Image heating device and image forming device

ABSTRACT

An image heating apparatus is provided that enables the entire width of a heat-producing medium to be heated uniformly and an excessive rise in temperature to be prevented, without making the configuration complex. In a fixing unit  19  in which this apparatus is applied, a fixing belt  112  is an annular member that has an inner peripheral surface and an outer peripheral surface, and produces heat through the action of magnetic flux. An exciting coil  120  is located in proximity to the outer peripheral surface of fixing belt  112,  and generates magnetic flux that acts upon fixing belt  112.  A suppression member  650  is located in proximity to inner peripheral surface of fixing belt  112,  and reduces, of the magnetic flux generated by exciting coil  120,  magnetic flux that acts upon a paper non-passage area of fixing belt  112.

TECHNICAL FIELD

The present invention relates to an image heating apparatus using anelectromagnetic induction heating scheme for fixing an unfixed image,and an image forming apparatus such as an electrophotographic apparatusor electrostatographic apparatus that uses that image heating apparatus.

BACKGROUND ART

An example of an image heating apparatus that uses an electromagneticinduction heating method is disclosed in Unexamined Japanese PatentPublication No. HEI 10-74009.

FIG.1 is an oblique drawing of an image heating apparatus disclosed inUnexamined Japanese Patent Publication No. HEI 10-74009, showing anexample of an image heating apparatus that uses a magnetic fluxabsorption member that absorbs magnetic flux.

In FIG. 1, reference number 1 indicates a metal sleeve that producesheat by means of induction heating. Metal sleeve 1 is mounted on, andsupported in a rotatable fashion by, the outer periphery of acylindrical guide 7. Reference number 2 indicates a pressure roller thatexerts pressure on metal sleeve 1. An unfixed toner image formed onrecording paper 8 is heat-fixed when recording paper 8 passes throughthe nip area (pressure area) between metal sleeve 1 and pressure roller2. Reference number 4 indicates an exciting coil that is installedinside of guide 7 and generates a high-frequency magnetic field, andreference numbers 6 a and 6 b indicate magnetic flux absorption membersthat are located on the outside of metal sleeve 1 and absorb magneticflux.

Recording paper 8 bearing an unfixed toner image is transported to thenip area in the direction indicated by the arrow S. A fixed toner imageis then formed on recording paper 8 by the heat of metal sleeve 1 andthe pressure between metal sleeve 1 and pressure roller 2. In thisexample, recording paper 8 is transported with a reference position atits right-hand side in FIG. 8, and if the paper width varies, theleft-hand side in FIG. 8 is a paper non-passage area.

As shown in FIG. 1, magnetic flux absorption member 6 b on the left-handside is configured so as to be capable of parallel movement in the axialdirection along a rail 5 through rotation of a motor 3.

When wide recording paper 8 is passed through, magnetic flux absorptionmember 6 b is placed in a position facing metal sleeve 1 without theintermediation of magnetic flux absorption member 6 a.

On the other hand, when narrow recording paper 8 is passed through,magnetic flux absorption member 6 b is moved to the rear of magneticflux absorption member 6 a as shown in FIG. 2. By this means, magneticflux reaching metal sleeve 1 from exciting coil 4 in the papernon-passage area is reduced. Therefore, the calorific value of the endsof metal sleeve 1 is suppressed.

Thus, the temperature rise in the paper non-passage area of metal sleeve1 is reduced according to the width of recording paper 8.

However, with the image heating apparatus shown in FIG. 1, in order toperform parallel movement of magnetic flux absorption member 6 b, thedistance between movable magnetic flux absorption member 6 b and metalsleeve 1 and the distance between magnetic flux absorption member 6 aand metal sleeve 1 are different, as shown in FIG. 2. Consequently, adifference tends to occur between the calorific values of the part wheremovable magnetic flux absorption member 6 b is facing metal sleeve 1 andthe part where magnetic flux absorption member 6 a is facing metalsleeve 1. Therefore, it is not easy to heat the entire width of metalsleeve 1 uniformly.

FIG. 3 is an oblique drawing of another image heating apparatusdisclosed in Unexamined Japanese Patent Publication No. HEI 10-74009,showing an example of an image heating apparatus that uses a magneticflux shielding plate as a means of reducing magnetic flux acting uponmetal sleeve 1.

In the conventional image heating apparatus shown in FIG. 3, a magneticflux shielding plate 9 is positioned so as to be in line with the innersurface of a holder 10 between metal sleeve 1 and exciting coil 4. Then,when narrow recording paper 8 is passed through, magnetic flux shieldingplate 9 is moved to a position where it covers exciting coil 4 over anaxial direction range equivalent to the paper non-passage area of metalsleeve 1, and when wide recording paper 8 is passed through, magneticflux shielding plate 9 is retracted to the outer edge of the paperpassage width of metal sleeve 1. Thus, the entire width of metal sleeve1 is heated uniformly when wide recording paper 8 is passed through.

However, in the image heating apparatus shown in FIG. 3, since magneticflux shielding plate 9 is installed so as to be in line with the innersurface of holder 10 between metal sleeve 1 and exciting coil 4,magnetic flux masking shield 9 must be made thin. When magnetic fluxshielding plate 9 is made thin, heat production due to induction heatingincreases. Moreover, as holder 10 is generally of a plastic materialwith low thermal conductivity, there is little heat dissipation frommagnetic flux shielding plate 9 to holder 10. There is consequently apossibility that magnetic flux shielding plate 9 will continue to risein temperature.

Furthermore, a problem with the image heating apparatus shown in FIG. 1is that a mechanism is necessary to perform parallel movement ofmagnetic flux absorption member 6 b, making the configuration of theoverall apparatus complex and large.

DISCLOSURE OF INVENTION

It is an object of the present invention to provide an image heatingapparatus that enables the entire width of a heat-producing member to beheated uniformly and an excessive rise in temperature to be prevented,without making the configuration complex.

According to one mode of the present invention, an image heatingapparatus has an annular heat-producing member that has a pair ofprincipal surfaces and produces heat through the action of magneticflux; a magnetic flux generation section that is located in proximity tothe first principal surface of the pair of principal surfaces andgenerates magnetic flux that acts upon the heat-producing member; and amagnetic flux reduction section that is located in proximity to thesecond principal surface of the pair of principal surfaces and reduces,of the magnetic flux generated by the magnetic flux generation section,magnetic flux that acts upon a paper non-passage area of theheat-producing member.

According to another mode of the present invention, an image heatingapparatus is equipped with an induction-heated thin heat-producingmember that transfers heat directly or indirectly to a heated mediumthat moves with an image; an excitation section that generates magneticflux and induction-heats the heat-producing member; a temperaturecontrol section that controls the excitation section and makes thetemperature of a contact surface that comes into contact with the heatedmedium a predetermined temperature; and a heat production adjustmentsection that is located on the opposite side of the heat-producingmember relative to the excitation section, and adjusts heat productiondistribution of the heat-producing member by adjusting the magnetic fluxacting upon the heat-producing member; wherein the heat productionadjustment section has an opposed core of ferromagnetic material whosetemperature varies according to the temperature of the heat-producingmember and whose Curie point is in the range of −10° C. to +100° C.relative to the maximum value of the predetermined temperature.

According to yet another mode of the present invention, an image heatingapparatus is equipped with an induction-heated thin heat-producingmember that transfers heat directly or indirectly to a heated mediumthat moves with an image; an excitation section that generates magneticflux and induction-heats the heat-producing member; a temperaturecontrol section that controls the excitation section and makes thetemperature of a fixing surface that comes into contact with the heatedmedium a predetermined temperature; and a heat production adjustmentsection that is located on the opposite side of the heat-producingmember relative to the excitation section, and adjusts heat productiondistribution of the heat-producing member by adjusting the magnetic fluxacting upon the heat-producing member; wherein the heat productionadjustment section has an opposed core of ferromagnetic material whosetemperature varies according to the temperature of the heat-producingmember and whose Curie point is in the range of 140° C. to 250° C.

According to a still further mode of the present invention, an imageheating apparatus is equipped with an induction-heated thinheat-producing member that transfers heat directly or indirectly to aheated medium that moves with an image; an excitation section thatgenerates magnetic flux and induction-heats the heat-producing member; atemperature control section that controls the excitation section andmakes the temperature of a fixing surface that comes into contact withthe heated medium a predetermined temperature; and a heat productionadjustment section that is located on the opposite side of theheat-producing member relative to the excitation section, and adjustsheat production distribution of the heat-producing member by adjustingthe magnetic flux acting upon the heat-producing member; wherein theheat production adjustment section has a switching section that performson/off switching of a suppression coil composed of an electricalconductor that is linked to the magnetic flux.

According to a still further mode of the present invention, an imageheating apparatus is equipped with an induction-heated thin, cylindricalheat-producing member that transfers heat directly or indirectly to aheated medium that moves with an image; an excitation section that facesthe outer peripheral surface of the heat-producing member, generatesmagnetic flux, and induction-heats the heat-producing member; atemperature control section that controls the excitation section andmakes the temperature of a fixing surface that comes into contact withthe heated medium a predetermined temperature; and a heat productionadjustment section that is located on the opposite side of theheat-producing member relative to the excitation section, and adjustsheat production distribution of the heat-producing member by adjustingthe magnetic flux acting upon the heat-producing member; wherein theheat production adjustment section has a rotatable opposed core offerromagnetic material whose cross-sectional shape varies in the axialdirection of the heat-producing member.

According to a still further mode of the present invention, an imageheating apparatus is equipped with an induction-heated thin, cylindricalheat-producing member that transfers heat directly or indirectly to aheated medium that moves with an image; an excitation section that facesthe outer peripheral surface of the heat-producing member, generatesmagnetic flux, and induction-heats the heat-producing member; atemperature control section that controls the excitation section andmakes the temperature of a fixing surface that comes into contact withthe heated medium a predetermined temperature; and a heat productionadjustment section that is located on the opposite side of theheat-producing member relative to the excitation section, and adjustsheat production distribution of the heat-producing member by adjustingthe magnetic flux acting upon the heat-producing member; wherein theheat production adjustment section has a rotatable opposed core whichconsists of divided ferromagnetic materials and has a cross-sectionalshape varying in the axial direction of the heat-producing member.

According to a still further mode of the present invention, an imageheating apparatus is equipped with an induction-heated thin, cylindricalheat-producing member that transfers heat directly or indirectly to aheated medium that moves with an image; an excitation section thatgenerates magnetic flux and induction-heats the heat-producing member; atemperature control section that controls the excitation section andmakes the temperature of a fixing surface that comes into contact withthe heated medium a predetermined temperature; and a heat productionadjustment section that is located on the opposite side of theheat-producing member relative to the excitation section, and adjustsheat production distribution of the heat-producing member by adjustingthe magnetic flux acting upon the heat-producing member; wherein theheat production adjustment section has a movable magnetic fluxsuppression member of low-resistivity material.

According to a still further mode of the present invention, an imageheating apparatus is equipped with an induction-heated thinheat-producing member that transfers heat directly or indirectly to aheated medium that moves with an image; an excitation section thatgenerates magnetic flux and induction-heats the heat-producing member; atemperature control section that controls the excitation section andmakes the temperature of a contact surface that comes into contact withthe heated medium a predetermined temperature; and a heat productionadjustment section that is located on the opposite side of theheat-producing member relative to the excitation section, and adjustsheat production distribution of the heat-producing member by adjustingthe magnetic flux acting upon the heat-producing member; wherein theheat production adjustment section has an opposed core of ferromagneticmaterial, and the Curie point of the opposed core is set higher than thetemperature of the opposed core in a paper passage area and lower thanthe temperature of the opposed core in a paper non-passage area.

According to a still further mode of the present invention, an imageheating apparatus is equipped with an induction-heated thinheat-producing member that transfers heat directly or indirectly to aheated medium that moves with an image; an excitation section thatgenerates magnetic flux and induction-heats the heat-producing member; atemperature control section that controls the excitation section andmakes the temperature of a contact surface that comes into contact withthe heated medium a predetermined temperature; and an opposed core offerromagnetic material that is located on the opposite side of theheat-producing member relative to the excitation section, and adjustsheat production distribution of the heat-producing member by adjustingthe magnetic flux acting upon the heat-producing member; wherein thedistance between the heat-producing member and the opposed core is setconstant in the area facing the exciting section.

According to a still further mode of the present invention, an imageheating apparatus is equipped with an induction-heated thinheat-producing member that transfers heat directly or indirectly to aheated medium that moves with an image; an excitation section thatgenerates magnetic flux and induction-heats the heat-producing member; atemperature control section that controls the excitation section andmakes the temperature of a contact surface that comes into contact withthe heated medium a predetermined temperature; and an opposed core offerromagnetic material that is located on the opposite side of theheat-producing member relative to the excitation section, and adjustsheat production distribution of the heat-producing member by adjustingthe magnetic flux acting upon the heat-producing member; wherein thedistance between the heat-producing member and the opposed core in apaper non-passage area is set greater than the distance between theheat-producing member and the opposed core in a paper passage area.

According to a still further mode of the present invention, an imageheating apparatus is equipped with an induction-heated thinheat-producing member that transfers heat directly or indirectly to aheated medium that moves with an image; an excitation section thatgenerates magnetic flux and induction-heats the heat-producing member; atemperature control section that controls the excitation section andmakes the temperature of a contact surface that comes into contact withthe heated medium a predetermined temperature; and an opposed core offerromagnetic material that is located on the opposite side of theheat-producing member relative to the excitation section, and adjustsheat production distribution of the heat-producing member by adjustingthe magnetic flux acting upon the heat-producing member; wherein thearea of the opposed core facing the heat-producing member in a papernon-passage area is set larger than the area of the opposed core facingthe heat-producing member in a paper passage area.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an oblique drawing showing an example of a conventional imageheating apparatus;

FIG. 2 is a side view of a magnetic flux absorption member installed inthe image heating apparatus in FIG. 1;

FIG. 3 is an oblique drawing showing another example of a conventionalimage heating apparatus;

FIG. 4 is a cross-sectional drawing showing an example of the generalconfiguration of an image forming apparatus that uses an image heatingapparatus according to Embodiment 1 of the present invention as a fixingunit;

FIG. 5 is a cross-sectional drawing of a fixing unit according toEmbodiment 1 of the present invention;

FIG. 6 is a rear view of a fixing unit viewed from the directionindicated by arrow G in FIG. 5;

FIG. 7 is a circuit diagram showing the basic configuration of theexciting circuit of a fixing unit according to Embodiment 1 of thepresent invention;

FIG. 8 is an explanatory drawing of the electromagnetic induction actionin a fixing unit according to Embodiment 1 of the present invention;

FIG. 9 is a cross-sectional drawing showing a first different sampleconfiguration of a fixing unit according to Embodiment 1 of the presentinvention;

FIG. 10 is a cross-sectional drawing showing a second different sampleconfiguration of a fixing unit according to Embodiment 1 of the presentinvention;

FIG. 11 is a cross-sectional drawing showing a third different sampleconfiguration of a fixing unit according to Embodiment 1 of the presentinvention;

FIG. 12 is a cross-sectional drawing showing a fourth different sampleconfiguration of a fixing unit according to Embodiment 1 of the presentinvention;

FIG. 13 is a cross-sectional drawing of the principal parts of a fixingunit according to Embodiment 2 of the present invention;

FIG. 14 is a principal-part configuration drawing of a magnetic fluxadjustment section viewed from the direction indicated by arrow G inFIG. 13;

FIG. 15 is a cross-sectional drawing of the principal parts of a fixingunit according to Embodiment 3 of the present invention;

FIG. 16 is an arrow-view drawing of a magnetic flux adjustment sectionviewed from the direction indicated by arrow H in FIG. 15;

FIG. 17 is a principal-part configuration drawing of a samplemodification of a magnetic flux adjustment section according toEmbodiment 3 of the present invention;

FIG. 18 is a principal-part configuration drawing of another samplemodification of a magnetic flux adjustment section according toEmbodiment 3 of the present invention;

FIG. 19A is a cross-sectional drawing of the principal parts of a fixingunit according to Embodiment 4 of the present invention, showing a casewhere magnetic flux acts upon the entire width of the fixing belt;

FIG. 19B is a cross-sectional drawing of the principal parts of a fixingunit according to Embodiment 4 of the present invention, showing a casewhere magnetic flux acting upon other than a narrow paper passage rangeof the fixing belt is decreased;

FIG. 19C is a cross-sectional drawing of the principal parts of a fixingunit according to Embodiment 4 of the present invention, showing a casewhere magnetic flux acting upon a narrow paper passage range of thefixing belt is decreased;

FIG. 20 is a principal-part configuration drawing of a magnetic fluxadjustment section viewed from the direction indicated by arrow H inFIG. 19C;

FIG. 21A is a cross-sectional drawing of the principal parts of a fixingunit according to Embodiment 5 of the present invention, showing a casewhere magnetic flux acts upon the entire width of the fixing belt;

FIG. 21B is a cross-sectional drawing of the principal parts of a fixingunit according to Embodiment 5 of the present invention, showing a casewhere magnetic flux acting upon other than a narrow paper passage rangeof the fixing belt is decreased;

FIG. 21C is a cross-sectional drawing of the principal parts of a fixingunit according to Embodiment 5 of the present invention, showing a casewhere magnetic flux acting upon other than a medium-width paper passagerange of the fixing belt is decreased;

FIG. 22 is a principal-part configuration drawing of a magnetic fluxadjustment section viewed from the direction indicated by arrow H inFIG. 21B;

FIG. 23 is a cross-sectional drawing of the principal parts of a fixingunit according to Embodiment 6 of the present invention;

FIG. 24 is a principal-part configuration drawing of a magnetic fluxadjustment section viewed from the direction indicated by arrow H inFIG. 23;

FIG. 25 is a cross-sectional drawing showing a first different sampleconfiguration of a fixing unit according to Embodiment 6 of the presentinvention;

FIG. 26 is a cross-sectional drawing showing a second different sampleconfiguration of a fixing unit according to Embodiment 6 of the presentinvention;

FIG. 27 is a cross-sectional drawing showing a third different sampleconfiguration of a fixing unit according to Embodiment 6 of the presentinvention;

FIG. 28 is a cross-sectional drawing showing a fourth different sampleconfiguration of a fixing unit according to Embodiment 6 of the presentinvention;

FIG. 29 is a cross-sectional drawing showing a fifth different sampleconfiguration of a fixing unit according to Embodiment 6 of the presentinvention;

FIG. 30 is a cross-sectional drawing of a fixing unit according toEmbodiment 7 of the present invention;

FIG. 31 is a principal-part configuration drawing of a magnetic fluxadjustment section in the fixing unit in FIG. 30;

FIG. 32 is a cross-sectional drawing showing a different sampleconfiguration of a fixing unit according to Embodiment 7 of the presentinvention; and

FIG. 33 is a principal-part configuration drawing of a magnetic fluxadjustment section in the fixing unit in FIG. 32.

BEST MODE FOR CARRYING OUT THE INVENTION

An image heating apparatus of the present invention is equipped with aninduction-heated thin heat-producing member that transfers heat directlyor indirectly to a heated medium that moves with an image; an excitationsection that generates magnetic flux and induction-heats theheat-producing member; a temperature control section that controls theexcitation section and makes the temperature of a contact surface thatcomes into contact with the heated medium a predetermined temperature;and a heat production adjustment section that is located on the oppositeside of the heat-producing member relative to the excitation section,and adjusts heat production distribution of the heat-producing member byadjusting the magnetic flux acting upon the heat-producing member;wherein the heat production adjustment section has an opposed core offerromagnetic material whose temperature varies according to thetemperature of the heat-producing member and whose Curie point is in therange of −10° C. to +100° C. relative to the maximum value of thepredetermined temperature.

By this means, it is possible to prevent an excessive temperature risewhereby a paper non-passage area becomes excessively hot, without amember that moves mechanically.

Also, at or below a predetermined temperature, magnetic coupling betweenthe heat-producing member and excitation member is good, and thereforethe efficiency of induction heating that heats the heat-producing memberis high. Moreover, magnetic flux distribution is continuously variablein accordance with axial direction temperature distribution by recordingpaper of any width. Furthermore, magnetic flux penetrating theheat-producing member can be prevented from leaking inside or outsidethe apparatus.

Also, an image heating apparatus of the present invention is equippedwith an induction-heated thin heat-producing member that transfers heatdirectly or indirectly to a heated medium that moves with an image; anexcitation section that generates magnetic flux and induction-heats theheat-producing member; a temperature control section that controls theexcitation section and makes the temperature of a fixing surface thatcomes into contact with the heated medium a predetermined temperature;and a heat production adjustment section that is located on the oppositeside of the heat-producing member relative to the excitation section,and adjusts heat production distribution of the heat-producing member byadjusting the magnetic flux acting upon the heat-producing member;wherein the heat production adjustment section has an opposed core offerromagnetic material whose temperature varies according to thetemperature of the heat-producing member and whose Curie point is in therange of 140° C. to 250° C.

By this means, it is possible to prevent an excessive temperature risewhereby a paper non-passage area becomes excessively hot without amember that moves mechanically. Also, at or below a predeterminedtemperature, magnetic coupling between the heat-producing member andexcitation member is good, and therefore the efficiency of inductionheating that heats the heat-producing member is high. Moreover, magneticflux distribution is continuously variable in accordance with axialdirection temperature distribution by recording paper of any width.Furthermore, magnetic flux penetrating the heat-producing member can beprevented from leaking inside or outside the apparatus.

It is desirable for the heat production adjustment section to be incontact with the heat-producing member or a member heated by theheat-producing member. By this means, the temperature of the magneticflux adjustment section changes more quickly in response to a change intemperature of the heat-producing member. Consequently, an excessivetemperature rise of the heat-producing member can be prevented rapidly.

The heat production adjustment section is facing and at a distance fromthe heat-producing member or a member heated by the heat-producingmember, and it is desirable for this distance to be not less than 0.3 mmand not more than 2 mm. By this means, the temperature of the magneticflux adjustment section changes more quickly in response to a change intemperature of the heat-producing member. Consequently, an excessivetemperature rise of the heat-producing member can be prevented rapidly.Also, contact of a member facing the heat production adjustment membercan be prevented. Also, magnetic coupling between the heat-producingmember and excitation member is good, and therefore the efficiency ofinduction heating that heats the heat-producing member is high.

It is also desirable for the infrared emissivity of at least one of theopposed surfaces opposing and brought into proximity at theaforementioned distance to be not less than 0.8 and not more than 1.0.By this means, heat transfer by infrared rays is increased, and thetemperature of the magnetic flux adjustment section changes more quicklyin response to a change in temperature of the heat-producing member.Consequently, an excessive temperature rise of the heat-producing membercan be prevented rapidly.

An image heating apparatus of the present invention is equipped with aninduction-heated thin heat-producing member that transfers heat directlyor indirectly to a heated medium that moves with an image; an excitationsection that generates magnetic flux and induction-heats theheat-producing member; a temperature control section that controls theexcitation section and makes the temperature of a fixing surface thatcomes into contact with the heated medium a predetermined temperature;and a heat production adjustment section that is located on the oppositeside of the heat-producing member relative to the excitation section,and adjusts heat production distribution of the heat-producing member byadjusting the magnetic flux acting upon the heat-producing member;wherein the heat production adjustment section has a suppression coilcomposed of an electrical conductor that is linked to the magnetic flux,and a switching section that performs on/off switching of thesuppression coil.

By this means, it is possible to prevent an excessive temperature risewhereby a paper non-passage area becomes excessively hot, without amember that moves mechanically. Also, installation on the opposite sideof the heat-producing member means that the current and voltage inducedin the suppression coil are small. By this means, it is possible forheat production of the suppression coil to be made small, and at thesame time for the withstand voltage and current capacity of theswitching section to be made small. As a result, an inexpensive andsimple configuration can be implemented.

Also, with regard to the heat production adjustment section, it isdesirable for an opposed core of high-permeability material throughwhich magnetic flux linked to the suppression coil passes to be locatedinside the suppression coil and/or on the opposite side with respect tothe heat-producing member.

By this means, magnetic coupling between the excitation section andsuppression coil is improved, and the action of the suppression coil dueto switching by the switching section is increased.

An image heating apparatus of the present invention is equipped with aninduction-heated thin, cylindrical heat-producing member that transfersheat directly or indirectly to a heated medium that moves with an image;an excitation section that faces the outer peripheral surface of theheat-producing member, generates magnetic flux, and induction-heats theheat-producing member; a temperature control section that controls theexcitation section and makes the temperature of a fixing surface thatcomes into contact with the heated medium a predetermined temperature;and a heat production adjustment section that is located on the oppositeside of the heat-producing member relative to the excitation section,and adjusts heat production distribution of the heat-producing member byadjusting the magnetic flux acting upon the heat-producing member;wherein the heat production adjustment section has a rotatable, unifiedopposed core of ferromagnetic material whose cross-sectional shapevaries in the axial direction of the heat-producing member.

By this means, it is possible to prevent an excessive temperature risewhereby a paper non-passage area becomes excessively hot, by rotatingthe unified opposed core, enabling the mechanical configuration to bemade simple and inexpensive, and at the same time enabling the apparatusto be made small. Also, the intensity of heat production distributioncan be varied arbitrarily by varying the rotational phase of the axis.

It is also desirable for the distance between the heat-producing memberand the opposed core to be fixed in the axial direction at least onepart in the circumferential direction of the opposed core. By thismeans, when this part is positioned to face the excitation section,uniform and highly efficient heating is possible.

It is also desirable for heat production distribution to be possiblewhereby the intensity of heat production distribution regulated by theopposed core reverses the intensity by rotation of the opposed core. Bymeans of this configuration, after only a narrow range has been heatedup when narrow paper is used, the low-temperature part outside thatrange can be heated intensively. By this means, when using narrow paper,the heating-up energy is small and at the same time a short-timeheating-up is possible. Also, uniform and high-quality images can beobtained even if wide paper is passed through immediately after-narrowpaper is passed through.

An image heating apparatus of the present invention is equipped with aninduction-heated thin, cylindrical heat-producing member that transfersheat directly or indirectly to a heated medium that moves with an image;an excitation section that faces the outer peripheral surface of theheat-producing member, generates magnetic flux, and induction-heats theheat-producing member; a temperature control section that controls theexcitation section and makes the temperature of a fixing surface thatcomes into contact with the heated medium a predetermined temperature;and a heat production adjustment section that is located on the oppositeside of the heat-producing member relative to the excitation section,and adjusts heat production distribution of the heat-producing member byadjusting the magnetic flux acting upon the heat-producing member;wherein the heat production adjustment section has a rotatable opposedcore which consists of divided ferromagnetic materials and has across-sectional shape varying in the axial direction of theheat-producing member.

By this means, the intensity of heat production distribution is variedfor each part by varying the rotational phase of the divided opposedcores, and therefore the flexibility of heat production distribution tobe set for a combination of the divided opposed cores is greater thanthat for the unified opposed core.

With regard to the opposed core, it is also desirable for at least onepart in the axial direction of the heat-producing member to be formed bycombining a plurality of materials of different permeability. By thismeans, adjustment of the calorific value can be performed by means ofboth rotational phase and material, enabling the possible heatproduction distribution intensity setting range to be extended. Also,the cross-sectional shape of the opposed core can be fixed in the axialdirection, enabling thermal capacity distribution inside theheat-producing member to be made uniform. By this means, uniformtemperature distribution of the heat-producing member can easily beachieved.

It is desirable for the opposed core to be formed by combining at leasta ferromagnetic material and a low-permeability electrical conductor. Bythis means, magnetic circuit variations due to opposed core rotation areincreased, extending the heat production distribution intensity controlrange. Also, inductive magnetic flux leakage can be suppressed.

It is also desirable for the electrical conductor to have a thickness ofnot less than 0.2 mm and not more than 3 mm in the radial direction ofthe heat-producing member. By this means, electrical conductor heatproduction can be prevented, and at the same time a small distance canbe set between the opposed core and heat-producing member, enabling themagnetic coupling of the induction heating section to be increased.

Furthermore, it is desirable for the cross-sectional shape of theopposed core to vary continuously in the axial direction in at least onepart of the axial direction of the heat-producing member. By this means,calorific value distribution can be adjusted continuously in the axialdirection by the angle of rotation of the opposed core. Consequently,the maximum necessary heat production area can be set for a plurality ofpaper widths.

According to a still further mode of the present invention, an imageheating apparatus is equipped with an induction-heated thin, cylindricalheat-producing member that transfers heat directly or indirectly to aheated medium that moves with an image; an excitation section thatgenerates magnetic flux and induction-heats the heat-producing member; atemperature control section that controls the excitation section andmakes the temperature of a fixing surface that comes into contact withthe heated medium a predetermined temperature; and a heat productionadjustment section that is located on the opposite side of theheat-producing member relative to the excitation section, and adjustsheat production distribution of the heat-producing member by adjustingthe magnetic flux acting upon the heat-producing member; wherein theheat production adjustment section has a movable magnetic fluxsuppression member of low-resistivity material.

By this means, heat production distribution control is possible withoutexpensive magnetic material. Also, magnetic flux penetrating theheat-producing member can be prevented from leaking inside or outsidethe apparatus. Furthermore, installation on the opposite side of theheat-producing member means that there is little magnetic flux actingupon the magnetic flux reduction section, and therefore magnetic fluxreduction member heat production is small. As a result, the efficiencyof induction heating that heats the heat-producing member is high.

It is desirable for an opposed core of ferromagnetic material to beprovided on the opposite side of the magnetic flux suppression memberrelative to the heat-producing member. By this means, the heatproduction distribution intensity control range through movement of themagnetic flux suppression member is extended.

It is also desirable for the magnetic flux suppression member to have athickness of 0.1 mm or above in the radial direction of theheat-producing member. By this means, the magnetic flux suppressionmember can prevent heat production due to inductive magnetic flux, andthe efficiency of induction heating that heats the heat-producing memberis increased.

An image forming apparatus of the present invention is equipped with anabove-described image heating apparatus, and the image heating apparatusfixes a toner image held on recording paper. By this means, the axialdirection calorific value can be controlled to any distribution.Therefore, it is possible to prevent an excessive temperature rise of apaper non-passage area with a simple and inexpensive configuration evenwhen narrow recording paper is used, and also to obtain a high-qualityfixed image when wide paper is used.

It is desirable for an image forming apparatus of the present inventionto be equipped with an above-described image heating apparatus; a firsttemperature sensor that is located in a range through which all kinds ofapplicable paper widths pass, and measures a temperature signal sent toa temperature control section; and a second temperature sensor that islocated in a range through which paper with the smallest applicablepaper width passes, and at least measures a temperature signal sent tothe heat production adjustment section; and for the heat productionadjustment section to adjust the heat production distribution of theheat-producing member based on a signal from the second temperaturesensor.

By this means, the axial direction calorific value can be adjustedrapidly to any distribution with respect to the temperature of theheat-producing member. Therefore, it is possible to prevent an excessivetemperature rise of a paper non-passage area with a simple andinexpensive configuration even when narrow recording paper is used, andalso to obtain a high-quality fixed image when wide paper is used.

With reference now to the accompanying drawings, embodiments of thepresent invention will be explained in detail below. In all thefollowing embodiments, a case is described in which an image heatingapparatus of the present invention is used as a fixing unit for fixingan unfixed image, and that fixing unit is used in an image formingapparatus such as an electrophotographic apparatus orelectrostatographic apparatus, for example.

Embodiment 1

FIG. 4 is a cross-sectional drawing showing an example of the generalconfiguration of an image forming apparatus that uses a fixing unitaccording to Embodiment 1 of the present invention. FIG. 5 is across-sectional drawing of the fixing unit according to this embodimentshown in FIG. 4, FIG. 6 is a rear view of a fixing unit according tothis embodiment viewed from the direction indicated by arrow G in FIG.5, FIG. 7 is a circuit diagram showing the basic configuration of theexciting circuit of a fixing unit according to this embodiment, FIG. 8is a drawing explaining the heat production action, and FIG. 9 throughFIG. 12 are cross-sectional drawings showing different sample modes of afixing unit according to this embodiment.

The configuration and operation of this apparatus will now be described.Reference number 11 indicates an electrophotographic photosensitive body(hereinafter referred to as “photosensitive drum”). The surface ofphotosensitive drum 11 is charged uniformly by a charger 12 whilephotosensitive drum 11 is rotated at a fixed peripheral velocity in thedirection indicated by the arrow.

Reference number 13 indicates a laser beam scanner. Laser beam scanner13 outputs a laser beam modulated in accordance with a time serieselectrical digital pixel signal of image information input from a hostapparatus such as an image reading apparatus or computer (not shown).The surface of photosensitive drum 11 uniformly charged as describedabove undergoes selective scanning exposure by this laser beam, wherebyan electrostatic latent image conforming to the image information isformed on the surface of photosensitive drum 11.

This electrostatic latent image is then supplied with powdered tonercharged by a developing device 14 that has a rotated developing roller14a, and is developed as a toner image.

Meanwhile, recording paper 16 is fed one after another from a paper feedsection 15. Recording paper 16 is transported at appropriate timingsynchronized with the rotation of photosensitive drum 11 throughregistration rollers 17 to a transfer section composed of photosensitivedrum 11 and a transfer roller 18 in contact with photosensitive drum 11.Through the agency of transfer roller 18 to which a transfer biasvoltage is applied, the toner image on photosensitive drum 11 issuccessively transferred to recording paper 16. After passing throughthe transfer section, recording paper 16 is separated fromphotosensitive drum 11 and input to fixing unit 19 functioning as animage heating apparatus, where fixing of the transferred toner image isperformed. Recording paper 16 on which an image has been fixed by thefixing process is output to an ejection tray 20.

After separation of recording paper 16, the surface of photosensitivedrum 11 is cleaned by having residual matter such as remainingtransferred toner removed by a cleaning apparatus 21, and is ready forthe next image forming operation.

In this embodiment, a center-based paper feed scheme is used—that is, amethod whereby both narrow paper and wide paper are transported withtheir center line in the width direction coinciding with the centerposition in the rotating axis direction of fixing unit 19.

Fixing unit 19 in the above-described image forming apparatus will nowbe described in detail. Reference number 112 indicates a fixing beltserving as a thin, endless heat-producing member. Fixing belt 112 ismade of polyimide resin in which conductive powder is dispersed toprovide electrical conductivity, and has a JIS (Japanese IndustrialStandards) −A25 degree, 150 μm silicone rubber layer laid upon a 45 mmdiameter, 100 μm thick base material surface, and a 20 μm thickfluororesin release layer further laid upon this silicone rubber layer.However, the configuration of fixing belt 112 is not limited to this.For example, heat-resistant fluororesin, PPS (polyphenylene sulfide), ora similar material in which conductive powder is dispersed, orelectroformed thin metal such as nickel or stainless steel, can be usedas the base material. Also, the surface release layer is not limited tofluororesin. For example, resin or rubber with good releasecharacteristics such as PTFE (polytetrafluoroethylene), PFA(tetrafluoroethylene-perfluoroalkylvinyl ether copolymer), FEP(fluorinated ethylene propylene copolymer), or the like, may be used,alone or mixed, for the surface release layer.

It is desirable for the thickness of the heat-producing layer to be morethan twice as thin as the skin depth corresponding to an inductionheating high-frequency current. If the heat-producing layer is thickerthan this, magnetic flux for induction heating will not penetrate theheat-producing member, and consequently the effect of the magnetic fluxadjustment section provided on the opposite side of the heat-producingmember from the excitation section will decrease.

Reference number 113 indicates a retaining roller. Retaining roller 113is made of a resin such as PPS, which is an insulating material, and hasa diameter of 20 mm and thickness of 0.3 mm. The outer peripheralsurface at either end of retaining roller 113 is supported in arotatable fashion by bearings (not shown). Ribs (not shown) to preventsnaking of fixing belt 112 are also provided at either end of retainingroller 113.

Reference number 114 indicates a 30 mm diameter low-thermal-conductivityfixing roller whose surface is of elastic foam silicone rubber of lowhardness (Asker C45 degrees).

Fixing belt 112 is suspended between retaining roller 113 and fixingroller 114 under predetermined tension, and is moved in the directionindicated by the arrow.

Reference number 115 indicates a pressure roller functioning as apressure section. Pressure roller 115 has an external diameter of φ30 mmand a surface of silicone rubber with a hardness of JIS−A60 degrees. Asshown in the drawing, pressure roller 115 presses against fixing belt112, forming a nip between pressure roller 115 and pressure roller 115.Pressure roller 115 is rotated by a drive section (not shown) of themain body of the apparatus. Fixing belt 112 and fixing roller 114 aredriven around by the rotation of pressure roller 115. To increase wearresistance and releasability, the surface of pressure roller 115 may becovered with PFA, PTFE, FEP, or similar rubber or resin, alone or mixed.

Reference number 120 indicates an exciting coil functioning as anexcitation section that induction-heats fixing belt 112. Theconfiguration of exciting coil 120 will be described in detail laterherein.

Reference number 116 indicates an opposed core (magnetic flux adjustmentsection) of a material (such as ferrite, for example) that hasinsulating properties and also has magnetic permeability and thermalconductivity of predetermined levels or higher. In this embodiment, thematerial of opposed core 116 is ferrite. Opposed core 116 is installedby being fixed to a spindle 117. For the ferrite material of opposedcore 116, the Curie point at which ferromagnetism is lost is set at 190°C. The clearance between opposed core 116 and the inner peripheralsurface of retaining roller 113 is 0.5 mm. Opposed core 116 of thisembodiment is of uniform cylindrical shape in its axial direction. Thefacing surfaces of opposed core 116 and retaining roller 113 are black.Reference number 119 indicates a toner image formed on recording paper16, and reference number 118 indicates a temperature sensor thatmeasures the temperature of fixing belt 112 for temperature control.

In fixing unit 19 of this embodiment, the maximum width of recordingpaper that can pass through is assumed to be the short side (297 mm inlength) of JIS standard A3 paper.

Reference number 120 indicates the exciting coil functioning as theexcitation section. Exciting coil 120 is formed of 9 turns of a wirebundle comprising 100 copper wires with an external diameter of 0.15 mmand an insulated surface.

As shown in FIG. 5 and FIG. 6, the wire bundle of exciting coil 120 isarranged in an arc shape along the outer peripheral surface of retainingroller 113 at the ends of retaining roller 113, and is placed along thebus line direction of that outer peripheral surface in other parts. Thewire bundle placed along the bus line direction is located on a virtualcylindrical surface with the rotation axis of retaining roller 113 asits center axis. At the edges of fixing belt 112, exciting coil 120 wirebundles are raised by being stacked in two rows.

Reference number 121 is an excitation core of ferrite ashigh-permeability material (with relative permeability of 2000, forexample). Excitation core 121 is composed of a center core 121 a locatedat the circulation center of exciting coil 120 and parallel to thecenter axis of fixing belt 112, approximately arch-shaped arch cores 121b located on the opposite side of exciting coil 120 relative to fixingbelt 112, and a pair of front cores 121 c located at the circulation endpart of exciting coil 120 and parallel to the rotation axis of fixingbelt 112. As shown in FIG. 6, a plurality of arch cores 121 b are spacedin the rotation axis direction of fixing belt 112. Center core 121 a islocated inside the aperture of the center part of circulated excitingcoil 120. Also, the pair of front cores 121 c are connected to eitherend of arch cores 121 b, and face fixing belt 112 without theintermediation of exciting coil 120. Center core 121 a, arch cores 121b, and front cores 121 c are magnetically coupled.

Apart from ferrite, a material of high magnetic permeability and highresistivity, such as ferrosilicon sheet, for example, is desirable asthe material of excitation core 121. Also, center core 121 a and frontcores 121 c may be divided into a plurality of parts in the lengthwisedirection.

Reference number 122 indicates a coil supporting member of PEEK(polyetheretherketone), PPS, or a similar resin with a highheat-resistant temperature. Exciting coil 120 and excitation core 121maintain the shape shown in the drawing by being attached to coilsupporting member 122.

FIG. 7 shows the basic circuit diagram of a monolithic resonance typeinverter used for an excitation circuit 123. An alternating current froma commercial power supply 160 is rectified by a rectifier circuit 161,and applied to a voltage resonance type inverter. In this inverter, ahigh-frequency current is applied to exciting coil 120 by switching of aswitching element 164 such as an IGBT (Insulated Gate BipolarTransistor) and a resonance capacitor 163. Reference number 162indicates a diode.

A 30 kHz alternating current with a maximum current amplitude of 60 Aand a maximum voltage amplitude of 600 V is applied to exciting coil 120from excitation circuit 123, a voltage resonance type inverter.Temperature sensor 118 is positioned in the center of the rotation axisdirection of fixing belt 112, facing fixing belt 112. The alternatingcurrent applied to exciting coil 120 is controlled by a temperaturesignal from this temperature sensor 118 so that the surface of fixingbelt 112 becomes 170 degrees centigrade, which is the fixing settemperature.

In an image forming apparatus that has a fixing unit 19 configured asdescribed above, a toner image is formed on the outer surface ofphotosensitive drum 11 (see FIG. 4), and after this toner image has beentransferred to the surface of recording paper 16, a recorded image isobtained by feeding recording paper 16 into the nip area from thedirection indicated by the arrow shown in FIG. 4 and fixing the tonerimage on recording paper 16.

In this embodiment, above-described exciting coil 120 causes fixing belt112 to produce heat by means of electromagnetic induction. Thisoperation is described below using FIG. 8.

As shown by the dotted lines in FIG. 8, magnetic flux M generated byexciting coil 120 due to an alternating current from excitation circuit123 penetrates fixing belt 112 from front cores 121 c of excitation core121 and enters opposed core 116 inside retaining roller 113, and passesthrough the interior of opposed core 116 due to the magnetic propertiesof opposed core 116. Magnetic flux M then penetrates fixing belt 112again and enters center core 121 a of excitation core 121, passesthrough arch cores 121 b, and reaches front cores 121 c. This magneticflux M goes through its repeated generation and extinction due to thealternating current of excitation circuit 123. An induction currentgenerated by variations of this magnetic flux M flows inside fixing belt112 and generates Joule heat. Center core 121 a and front cores 121 cconsecutive in the fixing belt 112 rotation axis direction have aneffect of scattering magnetic flux M that has passed through arch cores121 b in the rotation axis direction and uniformizing the magnetic fluxdensity.

Next, the operation of opposed core 116 will be described. When thetemperature of opposed core 116 is lower than the Curie point across theentire axial-direction width, opposed core 116 has ferromagnetismuniformly in the axial direction, and increases the magneticpermeability of the area through which magnetic flux M passes. As themagnetoresistance of this area falls, magnetic coupling between excitingcoil 120 and fixing belt 112 improves. Therefore, fixing belt 112 can beefficiently heated uniformly in the axial direction. Consequently, thehigh-frequency current and voltage applied to exciting coil 120 can beset low when predetermined power is applied. As a result, inexpensiveelectronic parts with a low withstand voltage and low current capacitycan be used in excitation circuit 123.

On the other hand, when paper narrow in width is fed continuously inthis state, while the entire width is heated uniformly recording paper16 passes through only the center part, absorbing heat, and thereforethe temperature of the edges of fixing belt 112, which are papernon-passage areas, rises. As opposed core 116 is facing and close tothese fixing belt 112 edges whose temperature rises, the temperature ofthe ends of opposed core 116 also rises. Consequently, the temperatureof the ends of opposed core 116 becomes higher than the Curie point ofthe constituent material, ferromagnetism is lost, and permeabilitydecreases.

In this state, the magnetic coupling between exciting coil 120 andfixing belt 112 decreases at the edges, and the calorific valuedeclines. A further rise in the temperature of the edges of fixing belt112 can thus be prevented. As a result, fixing defects such as offsetarising when the edge temperature is too high can be prevented even ifwide paper is fed through after continuous feeding of narrow paper. Atthe same time, it is possible to prevent fixing unit 19 and the body ofthe apparatus from exceeding their respective heat-resistanttemperatures and becoming deformed due to an excessive rise in thetemperature of fixing unit 19.

When the temperature of the edges of fixing belt 112 returns to a stateequivalent to the fixing temperature, the temperature of opposed core116 also falls to the Curie point or below and opposed core 116 isrestored to a ferromagnetic body, and therefore the initial state—thatis, a state of a high degree of magnetic coupling—is restored uniformlyin the axial direction.

Of course, the center part of fixing belt 112 has its heat absorbed byrecording paper 16 and is subjected to temperature control based on atemperature signal from temperature sensor 118, thereby being heldconstantly at a fixed temperature. Also, when maximum-width recordingpaper is passed through, the entire width is heated uniformly and heatis absorbed uniformly, so that extreme temperature distribution does notoccur.

In this embodiment, the Curie point of opposed core 116 (190° C.) ismade higher than the fixing temperature (170° C.), and therefore opposedcore 116 acts as a ferromagnetic body except in areas where thetemperature of fixing belt 112 becomes too high. Therefore, excitingcoil 120 and fixing belt 112 can be magnetically coupled efficientlywhen there is a rise in the fixing temperature or when wide paper is fedthrough. The above effect can be obtained when this Curie point is in arange of −10° C. to +100° C. relative to the maximum value of thepredetermined fixing temperature. When utilizing toner that uses commonstyrene acrylonitrile or polyester as a base, the above effect can beobtained when the Curie point is in the range of 140° C. to 250° C.

Actually, according to this embodiment, the Curie point of opposed core116 is set so as to be higher than the temperature of opposed core 116in a paper passage area and lower than the temperature of opposed core116 in a paper non-passage area. Therefore, in a paper passage area,fixing belt 112 can be uniformly heated efficiently by having themagnetic coupling between exciting coil 120 and fixing belt 112maintained, and in a paper non-passage area, the calorific value can bereduced and overheating of the fixing belt prevented by decreasing themagnetic coupling between exciting coil 120 and fixing belt 112.

Also, by locating high-permeability opposed core 116 within theinduction heating magnetic path, leakage of magnetic flux M outsidefixing unit 19 can be prevented.

Furthermore, as opposed core 116 has a uniform cross-sectional shape inthe axial direction, the calorific value of heat-producing areas inproximity to opposed core 116 is uniform in the axial direction.Therefore, uniform temperature distribution can easily be achieved byheating uniformly with exciting coil 120.

Moreover, by heating fixing belt 112 using a part wrapped aroundretaining roller 113 as a heat-producing section, the shape of fixingbelt 112 is stabilized and a constant distance can easily be maintainedbetween fixing belt 112 and exciting coil 120.

Also, as opposed core 116 is located inside retaining roller 113 thatrotates in contact with fixing belt 112, opposed core 116 is not cooledby heat dissipation. Therefore, the temperature of opposed core 116rises rapidly and with good responsiveness in accordance with a rise inthe temperature of fixing belt 112, enabling an excessive temperaturerise of fixing belt 112 to be prevented promptly.

As described above, according to this embodiment, it is possible toprevent the temperature of both ends where heat is not absorbed bynarrow recording paper 16 from rising excessively, and a component part(such as fixing unit 19, for example) of an image forming apparatus frombeing heated in excess of its heat-resistant temperature and beingdamaged or degraded, without using a mechanical configuration for ameans of heat production distribution adjustment. Furthermore, thetemperature distribution does not vary greatly across the entire widthof fixing belt 112 even when maximum-width recording paper is fedthrough immediately after narrow paper has been fed throughcontinuously, enabling the occurrence of fixing defects such as offsetto be prevented even when using wide paper.

With a conventional fixing unit, if the temperature of both ends becomesexcessively high when narrow paper is fed through continuously, it isnecessary to stop the printing operation and wait until the temperatureof both ends falls, or to increase the recording paper feeding interval,but with this embodiment, a rise in the temperature of both ends whennarrow paper is fed through can be suppressed, making it unnecessary towait or increase the paper feeding interval in the event of an excessiverise in temperature. Therefore, throughput (the number of sheets outputper unit time) can be set high when outputting narrow papercontinuously.

In this embodiment, opposed core 116 is assumed to be cylindrical.However, as long as the distance from fixing belt 112 opposite excitingcoil 120 is uniform in the axial direction, the cross-sectional shape ofopposed core 116 is not limited to this, and may be semicircular orarc-shaped. If the cross-sectional shape of opposed core 116 issemicircular or arc-shaped, the calorific value of opposed core 116 isless than when opposed core 116 is cylindrical, and therefore thetemperature of opposed core 116 changes more quickly in response to arise in the temperature of fixing belt 112.

In this embodiment, the distance between opposed core 116 and retainingroller 113 is taken to be 0.5 mm, but it is desirable for this distanceto be in the range of 0.3 mm to 2 mm. If the distance is less than this,nonuniformity of heat transfer distribution may occur in the axialdirection due to partial contact of retaining roller 113 and opposedcore 116. As a result, nonuniformity of temperature distribution mayoccur despite uniform heating, preventing a uniform fixed image frombeing obtained. On the other hand, if this distance exceeds the aboverange, thermal conductivity from fixing belt 112 and retaining roller113 to opposed core 116 will degrade, and the responsiveness of atemperature rise of opposed core 116 when the temperature of fixing belt112 rises will be poor. In practice, this distance may be 2 mm or less.

Also, it is desirable for the distance between opposed core 116 andfixing belt 112 to be 2 mm or less. If this distance exceeds 2 mm, themagnetic coupling between exciting coil 120 and fixing belt 112 will bepoor, and it may not be possible to perform induction heatingefficiently.

In this embodiment, the mutually facing surfaces of opposed core 116 andretaining roller 113 are both black, facilitating the emission andabsorption of infrared rays between opposed core 116 and retainingroller 113. This facilitates the transfer of heat between the two. Thisis practicable if at least one of the opposing surfaces has infraredemissivity of 0.8 to 1.0, and it is desirable for the infraredemissivity of both opposing surfaces to be 0.8 or above. As infrared rayemissivity and absorptivity are the same numeric value, settingemissivity high means simultaneously setting absorptivity high.

In this embodiment, opposed core 116 is fixed, but it may also beconfigured so as to rotate integrally with retaining roller 113.

Also, the configuration of the heat-producing part is not limited to theseparate installation of retaining roller 113 and opposed core 116 asdescribed above. For example, the same kind of effect can also beachieved with a configuration in which opposed core 116 has a rollershape and rotates itself suspending fixing belt 112 directly, as shownin FIG. 9. In this case it is not necessary to provide retaining roller113, simplifying the configuration. Also, since heat is transferreddirectly from fixing belt 112 to opposed core 116, the temperature ofopposed core 116 changes more quickly in response to a rise in thetemperature of fixing belt 112.

Furthermore, with regard to the configuration of the heat-producingpart, exciting coil 120 and opposed core 116 may be installed so as tosandwich fixing belt 112 stretched over retaining roller 113 and fixingroller 114, as shown in FIG. 10.

Moreover, the configuration of fixing unit 19 is not limited to havingfixing belt 112 as described above suspended on two rollers (retainingroller 113 and fixing roller 114) and having exciting coil 120 facingthe outer peripheral surface of fixing belt 112. For example, aconfiguration may also be implemented in which exciting coil 120 isprovided inside retaining roller 113, retaining roller 113 is pressedagainst pressure roller 115 via fixing belt 112, and an approximatelyarc-shaped opposed core 116 is facing and close to the outer peripheralsurface of fixing belt 112, as shown in FIG. 11.

A configuration may also be implemented in which a fixing belt 112 ofthe same diameter encloses the outer periphery of retaining roller 113,and retaining roller 113 is pressed against pressure roller 115 viafixing belt 112. With this configuration, it is not necessary to providefixing roller 114 and retaining roller 113 separately, and a mechanismproviding tension to fixing belt 112 is also unnecessary, enabling theconfiguration to be simplified and manufacturing costs to be reduced. Inaddition, the peripheral length of fixing belt 112 is shortened,reducing the thermal capacity for a rise in temperature, and thusdecreasing the energy necessary for a temperature rise and at the sametime enabling the temperature rise time to be shortened.

Embodiment 2

FIG. 13 is a cross-sectional drawing of the principal parts of a fixingunit according to Embodiment 2 of the present invention, and FIG. 14 isa principal-part configuration drawing of opposed core 116 constitutinga magnetic flux adjustment section viewed from the direction indicatedby arrow G in FIG. 13.

This embodiment differs from Embodiment 1 in the configuration of themagnetic flux adjustment section. That is to say, in this embodiment, ateither end of opposed core 116, a 2-turn short coil (hereinafterreferred to as “suppression coil”) 230 of litz wire is provided in thepart facing exciting coil 120. Also provided are relays 231 serving asswitching sections that perform electrical on/off switching of eitherend of opposed core 116. Relays 231 have a switching element such as apower transistor and a contact. The cross-sectional shape of opposedcore 116 is uniformly semicircular in the axial direction. Opposed core116 is supported fixedly and not rotated. A temperature sensor 232 isprovided that measures the temperature of fixing belt 112 outside thenarrow paper passage range and within the maximum-width paper passagerange, and relays 231 are opened and closed based on a temperaturesignal from temperature sensor 232.

Other details are similar to Embodiment 1, and configuration elements inthis embodiment that have the same action as in Embodiment 1 areassigned the same reference codes as in Embodiment 1, and detaileddescriptions thereof are omitted.

If the temperature measured by temperature sensor 232 is lower than afirst predetermined temperature (for example, 180° C.) that is higherthan the fixing temperature (for example, 170° C.), each relay 231 isplaced in the open state. In this state, current does not flow insuppression coil 230, and therefore fixing belt 112 is heated withuniform heat production distribution by exciting coil 120.

On the other hand, if the temperature measured by temperature sensor 232is higher than 180° C. due to continuous passage of narrow paper, eachrelay 231 is placed in the conducting state. In this state, an inductioncurrent flows in the direction in which it cancels out variations inmagnetic flux linked to suppression coil 230. Therefore, magnetic fluxcan no longer pass through the interior of suppression coil 230.Consequently, magnetic flux from exciting coil 120 acting upon fixingbelt 112 in the area in which suppression coil 230 is located decreases.As a result, heat production distribution of narrow paper non-passageareas declines, and an excessive rise in the temperature of the papernon-passage areas can be prevented.

Then, when the temperature measured by temperature sensor 232 becomes asecond predetermined temperature (for example, 160° C.) that is lowerthan the fixing temperature, relays 231 are placed in the open state anduniform heat production distribution is restored.

As an opposed core 116 on the opposite side of suppression coil 230 fromfixing belt 112 is used, the magnetic coupling of exciting coil 120,fixing belt 112, and suppression coil 230 improves, enabling thetemperature distribution adjustment action of suppression coil 230 bymeans of the opening and closing of relay 231 to be made sufficientlygreat. By locating part of opposed core 116 inside suppression coil 230,the temperature distribution adjustment action of suppression coil 230by means of the opening and closing of relay 231 can be furtherincreased.

As described above, according to this embodiment, the temperaturedistribution of fixing belt 112 can be kept constantly substantiallyuniform even when narrow paper is continuously fed through withoutproviding a mechanical contrivance. Therefore, fixing defects such ascold offset or hot offset due to nonuniformity of fixing temperaturedistribution can be prevented when wide paper is fed through immediatelyafter narrow paper is fed through, or when narrow paper and wide paperare fed through alternately. That is to say, if the entire width offixing belt 112 is heated during warm-up, it is possible to feed throughboth narrow paper and wide paper immediately. On the other hand, if onlythe narrow paper passage range, for example, is heated during warm-up,in the event of an abnormality such as non-rotation of fixing belt 112,the surface temperature of fixing belt 112 will rise steeply and asafety mechanism (such as a thermostat, for example) may not be able tofollow. If full-width heating is performed during warm-up, thetemperature rise time of fixing belt 112 can be extended, and thefollow-up operation of the safety mechanism can be assured.

Although it is also possible for suppression coil 230 to be locatedinside or close to exciting coil 120, in this embodiment suppressioncoil 230 is located on the opposite side of fixing belt 112 fromexciting coil 120. By this means, the current and voltage induced insuppression coil 230 are low, and a rise in the temperature ofsuppression coil 230 is suppressed. As a result, inexpensive materialwith a low withstand voltage and heat-resistant temperature can be usedfor the wire insulating coating. Also, inexpensive items with a lowwithstand voltage and current capacity can be used for relays 231 thatopen and close suppression coils 230. Furthermore, electromagnetic noisegenerated during relay 231 opening and closing operations can besuppressed.

An opposed core 116 located on the opposite side of suppression coil 230relative to fixing belt 112 is used, but a configuration in which noopposed core 116 is provided can also be implemented. In this case, theuse of an expensive and heavy material such as ferrite is not necessary,enabling the apparatus to be made less expensive and lighter.

Suppression coil 230 is not limited to an above-described wire materialwound around a plurality of times. For example, the same kind of effectcan be obtained with a configuration in which a thin metal sheet isformed with a single loop. This configuration does not require multiplewindings of wire, enabling the manufacturing process to be simplified.

It is not absolutely necessary for the installation range of suppressioncoil 230 to provide for the width of narrow paper fed through. Forexample, setting may be performed in a range greater than the width ofnarrow paper and less than the maximum paper width, taking intoconsideration the amount of heat lost by heat transfer from both endsvia the bearings.

Suppression coil 230 may have any configuration, as long as its loopformation direction is linked to magnetic flux from exciting coil 120.

Embodiment 3

FIG. 15 is a cross-sectional drawing of the principal parts of a fixingunit according to Embodiment 3 of the present invention, and FIG. 16 isa principal-part configuration drawing of opposed core 116 constitutinga magnetic flux adjustment section viewed from the direction indicatedby arrow H in FIG. 15.

This embodiment differs from Embodiment 2 in the configuration of themagnetic flux adjustment section. That is to say, in this embodiment,suppression coil 230 is not provided, and the cross-sectional shape of apart of cylindrical opposed core 116 corresponding to a narrow papernon-passage area is varied in the axial direction. Also, a gear 335 isfitted to the right-hand end of opposed core 116 in FIG. 16. A rotationsection 336 rotates this gear 335, and opposed core 116 rotates inaccordance with this rotation. A disk 337 that has a notch is fitted tothe other end (in FIG. 16, the left-hand end) of opposed core 116. Aphotosensor 338 is provided to detect the rotation of this notch.Rotation section 336 (in other words, the rotation of opposed core 116)is controlled based on a temperature signal from temperature sensor 232that measures the temperature of fixing belt 112 outside the narrowpaper passage range and within the maximum-width paper passage range.

Other details are similar to Embodiment 2, and configuration elements inthis embodiment that have the same action as in Embodiment 2 areassigned the same reference codes as in Embodiment 2, and detaileddescriptions thereof are omitted.

Opposed core 116 has a semicircular shape at both axial-direction ends(outside the narrow paper passage range), and a circular shape in theaxial-direction center part (within the narrow paper passage range). Thephases of the semicircular shapes at both ends coincide with respect tothe rotation axis, and the semicircular shapes are uniform in the axialdirection. Hereinafter, in this embodiment, opposed core 116 that hasthis kind of shape is regarded as a combination of two semicylinders,one of which is called part a, and the other one part b. Part a is asemicylinder that has substantially the same width as the maximum-widthpaper passage range, and part b is a semicylinder that has substantiallythe same width as the narrow paper passage range. Rotation section 336has a stepping motor. Rotation section 336 detects the origin of postureof opposed core 116 by means of a photosensor 338 signal, and sets theangle of rotation from this origin of posture as a number of steppingmotor drive pulses. There is no need to use an expensive detectionapparatus such as an expensive, high-resolution encoder for opposed core116, making the configuration simple and inexpensive.

The operation and action of opposed core 116 as a magnetic fluxadjustment section in this embodiment will now be described.

If the temperature measured by end temperature sensor 232 is lower thana first predetermined temperature (for example, 180° C.) that is higherthan the fixing temperature (for example, 170° C.), part a of opposedcore 116 is positioned to face exciting coil 120. When current is passedthrough exciting coil 120 in this state, magnetic flux acts uniformlyupon the entire axial-direction width of fixing belt 112, and inductionheating is performed uniformly. When recording paper 16 fed through iswide, heat is absorbed over substantially the entire width, andtherefore the temperature of fixing belt 112 is maintained uniformlyover its entire width.

When narrow recording paper 16 is passed through, heat of only thecenter part is absorbed by the recording paper, and together with this,temperature control is performed based on a temperature signal fromtemperature sensor 118 in proximity to the center part. Therefore, thetemperature of both end parts, which are paper non-passage areas, rises.When the temperature measured by temperature sensor 232 exceeds 180° C.,opposed core 116 is rotated and part b is positioned to face excitingcoil 120. In this state, the distance between a part of fixing belt 112corresponding to a paper non-passage area and opposed core 116 becomesgreater than the distance from the part corresponding to the centerpaper passage area. Consequently, the magnetic coupling between fixingbelt 112 and exciting coil 120 in the paper non-passage areas becomespoorer than for the paper passage area, and magnetic flux from excitingcoil 120 acting on fixing belt 112 in the paper non-passage areasdecreases. As a result, heat production distribution of the narrow papernon-passage areas declines, and an excessive rise in the temperature ofthe paper non-passage areas can be prevented.

Then, when the temperature measured by temperature sensor 232 becomes asecond predetermined temperature (for example, 160° C.) that is lowerthan the fixing temperature, part a of opposed core 116 is positioned toface exciting coil 120 and uniform heat production distribution isrestored.

As described above, according to this embodiment, the temperaturedistribution of fixing belt 112 can be kept constantly substantiallyuniform even when narrow paper is continuously fed through. Therefore,fixing defects such as cold offset or hot offset due to nonuniformity offixing temperature distribution can be prevented when wide paper is fedthrough immediately after narrow paper is fed through, or when narrowpaper and wide paper are fed through alternately. That is to say, if theentire width of fixing belt 112 is heated during warm-up, it is possibleto feed through both narrow paper and wide paper immediately. On theother hand, if only the narrow paper passage range, for example, isheated during warm-up, in the event of an abnormality such asnon-rotation of fixing belt 112, the surface temperature of fixing belt112 will rise steeply and a safety mechanism (such as a thermostat, forexample) may not be able to follow. If full-width heating is performedduring warm-up, the temperature rise time of fixing belt 112 can beextended, and the follow-up operation of the safety mechanism can beassured.

With a conventional fixing unit, if the temperature of both ends becomesexcessively high when narrow paper is fed through continuously, it isnecessary to stop the printing operation and wait until the temperatureof both ends falls, or to increase the recording paper feeding interval.With this embodiment, on the other hand, a rise in the temperature ofboth ends when narrow paper is fed through can be suppressed, making itunnecessary to wait or increase the paper feeding interval in the eventof an excessive rise in temperature. Therefore, throughput (the numberof sheets output per unit time) can be set high when outputting narrowpaper continuously.

As opposed core 116 is rotated as an integral unit, the mechanism forrotational drive is simple. In the case of a configuration in which thecenter part of the opposed core is fixed, a complex mechanism isnecessary in order to rotate only the end parts. Also, as opposed core116 is rotated inside retaining roller 113, the heat-producing sectioncan be made small.

In this embodiment, opposed core 116 is rotated (reversed) by 180degrees in order to adjust the heat production distribution of the ends.However, this angle of rotation is not limited to 180 degrees. Forexample, the angle of rotation may be adjusted in accordance withtemperature variations of paper non-passage areas. With thisconfiguration, the heat production distribution of paper non-passageareas can be controlled precisely, and the temperature distribution offixing belt 112 can be made uniform.

In this embodiment, the cross-sectional shape of the ends of opposedcore 116 is uniform in the axial direction. However, the cross-sectionalshape of opposed core 116 may be varied continuously in the rangecorresponding to narrow paper non-passage areas, as shown in FIG. 17.With this configuration, opposed core 116 has a semicircularcross-section only at the ends, and its cross-sectional shape variescontinuously until becoming a circular cross-section in the areacorresponding to the narrow paper passage range. That is to say, withthis opposed core 116, the distance from fixing belt 112 is greater thenearer the ranges in which the surface recedes from the fixedcylindrical surface in the rotation center direction are toaxial-direction end parts, and one of these receding ranges starts fromthe same bus-line in the circumferential direction.

When the above configuration is used, by varying the rotational phase ofopposed core 116 it is possible to vary continuously and arbitrarily thelength of the part where opposed core 116 is facing exciting coil 120 atthe same distance as the center part. By this means, the width of areaswith low heat production distribution at both ends can be setcontinuously and arbitrarily. As a result, the above-described effectcan be obtained for any width of recording paper 16 fed through.

In this embodiment, a member is not specially provided in the concavityfrom the cylindrical shape of opposed core 116, but an adjustment member338 of different permeability from that of opposed core 116 may beprovided in this part, as shown in FIG. 18.

If a magnetic material of lower permeability than opposed core 116 (suchas ferrite resin with a permeability of 10, for example) is used foradjustment member 338, the difference in peak intensity of the calorificvalue can be adjusted arbitrarily in accordance with the permeabilitiesof opposed core 116 and adjustment member 338.

Also, if a nonmagnetic conductive material such as aluminum or copper isused for adjustment member 338, the difference in peak intensity of thecalorific value can be further increased. This is because conductivematerial has a property of being susceptible to the flow of an eddycurrent in an induced magnetic field and scarcely allowing the passageof induced magnetic flux inside. Furthermore, as opposed core 116 has auniform cross-sectional shape in the axial direction, thermal capacitydistribution of the heat-producing section approaches uniformity in theaxial direction. Therefore, uniform temperature distribution can easilybe achieved by performing heating uniformly by means of exciting coil120.

Instead of varying the cross-sectional shape of opposed core 116continuously from the center part toward the ends, the cross-sectionalshape may be varied stepwise taking the recording paper widths used intoconsideration. According to this configuration, recording paper of aplurality of widths can be provided for, and the difference in calorificvalues at the boundary of a heated part and unheated part (strong andweak heat production distribution areas) can be made prominent.

Embodiment 4

FIG. 19A, FIG. 19B, and FIG. 19C are cross-sectional drawings of theprincipal parts of a fixing unit according to Embodiment 4 of thepresent invention, and FIG. 20 is a principal-part configuration drawingof opposed core 116 constituting a magnetic flux adjustment sectionviewed from the direction indicated by arrow H in FIG. 19C.

This embodiment differs from Embodiment 3 in the configuration of themagnetic flux adjustment section. That is to say, in this embodiment,opposed core 116 has three areas: A, B, and C. Areas A, B, and C aredefined by dividing opposed core 116 into three equal parts with threeplanes extending outward from spindle 117 as boundaries. The shape ofopposed core 116 is different in each of areas A, B, and C. In area A,opposed core 116 spans the full width of the axial direction. In area B,opposed core 116 spans only a range corresponding to the center narrowpaper passage area (narrow paper passage range). In area C, opposed core116 spans only ranges corresponding to the narrow paper non-passageareas at both ends (excluding the narrow paper passage range).

Other details are similar to Embodiment 3, and configuration elements inthis embodiment that have the same action as in Embodiment 3 areassigned the same reference codes as in Embodiment 3, and detaileddescriptions thereof are omitted.

The operation and action of opposed core 116 as a magnetic fluxadjustment section in this embodiment will now be described using FIG.19A, FIG. 19B, and FIG. 19C.

If the temperature difference between center temperature sensor 118 andend temperature sensor 232 is less than a predetermined temperaturedifference (for example, 15° C.), and the temperature measured bytemperature sensor 232 is lower than a first predetermined temperature(for example, 180° C.) that is higher than the fixing temperature (forexample, 170° C.), area A of opposed core 116 is positioned to faceexciting coil 120 as shown in FIG. 19A. When parts of areas B and C arealso facing exciting coil 120, the facing ranges of areas B and C aremade the same. When current is passed through exciting coil 120 in thisstate, magnetic flux acts uniformly upon the entire axial-directionwidth of fixing belt 112, and induction heating is performed uniformly.When recording paper 16 fed through is wide, heat is absorbed oversubstantially the entire width, and therefore the temperature of fixingbelt 112 is maintained uniformly over its entire width.

When narrow recording paper 16 is passed through, in the state shown inFIG. 19A, heat of only the center part is absorbed by recording paper16, and together with this, temperature control is performed based on atemperature signal from temperature sensor 118 in proximity to thecenter part. Therefore, the temperature of both end parts, which arepaper non-passage areas, rises. When the temperature measured bytemperature sensor 232 exceeds 180° C., opposed core 116 is rotated andarea B and part of area A are positioned to face exciting coil 120 asshown in FIG. 19B. In the state in which mainly area B is facingexciting coil 120, the distance between a part of fixing belt 112corresponding to a paper non-passage area and opposed core 116 becomesgreater than the distance from the part corresponding to the centerpaper passage area. Consequently, the magnetic coupling between fixingbelt 112 and exciting coil 120 in the paper non-passage areas becomespoorer than for the paper passage area, and magnetic flux from excitingcoil 120 acting upon fixing belt 112 in the paper non-passage areasdecreases. As a result, heat production distribution of the narrow papernon-passage areas declines, and an excessive rise in the temperature ofthe paper non-passage areas can be prevented.

Then, when the temperature measured by temperature sensor 232 becomes asecond predetermined temperature (for example, 160° C.) that is lowerthan the fixing temperature, area A is positioned to face exciting coil120 as shown in FIG. 19A, and uniform heat production distribution isrestored.

If a printing operation is performed with narrow paper when fixing unit19 is cold (for example, at room temperature), heating is started in thestate shown in FIG. 19B in order to heat only the center part. In thiscase, since only the center part is heated, the thermal capacity forheat production decreases. Therefore, the temperature can be raised tothe predetermined temperature (170° C.) with a small amount of energy,and if heating is performed with the same power, the temperature can beraised in a short time.

In this case, the temperature of fixing belt 112 in the papernon-passage areas does not rise to the fixing temperature, and it istherefore possible to prevent the temperature of pressure roller 115 inthe paper non-passage areas from becoming excessively higher than in thepaper passage area.

Furthermore, in this case a state is established in which thetemperature of center temperature sensor 118 is higher than that of endtemperature sensor 232. If wide paper is next to be fed through in thisstate, it is necessary to heat only both end parts. In this case, area Cand part of area A are positioned to face exciting coil 120 as shown inFIG. 19C. In the heat production distribution in this state, thecalorific value of the center part is small, and the calorific value ofthe end parts is large. By this means it is possible to change from astate in which the temperature of the end parts is low to a state ofuniform temperature distribution. At this time, the temperature of thepaper non-passage areas of pressure roller 115 has not risenexcessively, and therefore, even when wide paper is fed through, it ispossible to prevent irregularities such as uneven glossiness of a fixedimage caused by nonuniformity of the temperature of pressure roller 115,enabling high-quality images to be obtained.

The state illustrated in FIG. 19C can be employed when the temperatureof center temperature sensor 118 shows at least a predeterminedtemperature difference (for example, 15° C.) from that of endtemperature sensor 232.

As described above, according to this embodiment, the temperaturedistribution of fixing belt 112 can be kept constantly substantiallyuniform even when narrow paper is continuously fed through. Therefore,fixing defects such as cold offset or hot offset due to nonuniformity offixing temperature distribution can be prevented when wide paper is fedthrough immediately after narrow paper is fed through, or when narrowpaper and wide paper are fed through alternately. That is to say, if theentire width of fixing belt 112 is heated during warm-up, it is possibleto feed through both narrow paper and wide paper immediately. On theother hand, if only the narrow paper passage range, for example, isheated during warm-up, in the event of an abnormality such asnon-rotation of fixing belt 112, the surface temperature of fixing belt112 will rise steeply and a safety mechanism (such as a thermostat, forexample) may not be able to follow. If full-width heating is performedduring warm-up, the temperature rise time of fixing belt 112 can beextended, and the follow-up operation of the safety mechanism can beassured.

Also, when starting up for narrow paper printing, it is possible to heatonly the center part, enabling the temperature to be raised with a smallamount of energy, and also enabling the temperature to be raised in ashort time if heating is performed with the same power. Furthermore,uniform temperature distribution can be restored even when thetemperature of the end parts has become too low relative to that of thecenter part through heat dissipation to the end parts or the like.

Moreover, as opposed core 116 is rotated as an integral unit, themechanism for rotational drive is simple.

Embodiment 5

FIG. 21A, FIG. 21B, and FIG. 21C are cross-sectional drawings of theprincipal parts of a fixing unit according to Embodiment 5 of thepresent invention, and FIG. 22 is a principal-part configuration drawingof opposed core 116 constituting a magnetic flux adjustment sectionviewed from the direction indicated by arrow H in FIG. 21B.

This embodiment differs from Embodiment 3 in the configuration of themagnetic flux adjustment section. That is to say, in this embodiment,opposed core 116 is composed of three opposed cores: 116 a, 116 b, and116 c. Opposed cores 116 a, 116 b, and 116 c are defined by dividing theentire axial-direction width of opposed core 116 into three equal parts.The width of opposed core 116 a corresponds to the narrow paper passagerange, the width of opposed core 116 b corresponds to the medium-widthpaper passage range excluding the narrow paper passage range, and thewidth of opposed core 116 c corresponds to the maximum-width paperpassage range excluding the medium-width paper passage range. Spindle117 of opposed core 116 is divided equally into three spindles, 117 a,117 b, and 117 c, corresponding to opposed cores 116 a, 116 b, and 116 crespectively, and opposed cores 116 a, 116 b, and 116 c are fixed tospindles 117 a, 117 b, and 117 c respectively. Gears 540 a, 540 b, and540 c that rotate spindles 117 a, 117 b, and 117 c respectively are alsoprovided.

Opposed core 116 a is a combination of a part D and part d that eachhave a semicylindrical shape. Part D and part d are made of ferrite withdifferent permeabilities, with part D having a higher permeability thanpart d. Similarly, opposed core 116 b is a combination of a part E andpart e that each have a semicylindrical shape. Part E and part e aremade of ferrite with different permeabilities, with part E having ahigher permeability than part e. Similarly, opposed core 116 c is acombination of a part F and part f that each have a semicylindricalshape. Part F and part f are made of ferrite with differentpermeabilities, with part F having a higher permeability than part f.

Also, in this embodiment, the recording paper 16 paper passage referenceposition is the right-hand edge in FIG. 22, so that when narrowrecording paper 16 is fed through the left-hand side is a papernon-passage area. Furthermore, temperature sensor 118 for temperaturecontrol is installed in the narrow paper passage range, a temperaturesensor 541 is installed in the medium-width paper passage range outsidethe narrow paper passage range, and a temperature sensor 542 isinstalled in the maximum-width paper passage range outside themedium-width paper passage range.

Other details are similar to Embodiment 3, and configuration elements inthis embodiment that have the same action as in Embodiment 3 areassigned the same reference codes as in Embodiment 3, and detaileddescriptions thereof are omitted.

The operation and action of opposed core 116 as a magnetic fluxadjustment section in this embodiment will now be described using FIG.21A, FIG. 21B, and FIG. 21C.

If the temperature difference between temperature sensor 118 andtemperature sensors 541 and 542 is less than a predetermined temperaturedifference (for example, 15° C.), and the temperature measured bytemperature sensors 541 and 542 is lower than a first predeterminedtemperature (for example, 180° C.) that is higher than the fixingtemperature (for example, 170° C.), part D, part E, and part F ofopposed core 116 are positioned to face exciting coil 120 as shown inFIG. 21A. When current is passed through exciting coil 120 in thisstate, magnetic flux acts uniformly upon the entire axial-directionwidth of fixing belt 112, and induction heating is performed uniformly.When recording paper 16 fed through is wide, heat is absorbed oversubstantially the entire width, and therefore the temperature of fixingbelt 112 is maintained uniformly over its entire width.

When narrow paper is passed through in the state shown in FIG. 21A, heatof only the right-hand end part (the narrow paper passage range) isabsorbed by the recording paper, and together with this, temperaturecontrol is performed based on a temperature signal from narrow paperpassage range temperature sensor 118. Therefore, the temperature of thepaper non-passage area (the range comprising the maximum-width paperpassage range minus the narrow paper passage range) rises. When thetemperature measured by temperature sensors 541 and 542 exceeds 180° C.,opposed cores 116 b and 116 c are rotated through 180 degrees and partD, part e, and part f are positioned to face exciting coil 120 as shownin FIG. 21B. As the permeability of part e and part f is lower than thatof part D, the magnetic coupling between fixing belt 112 and excitingcoil 120 in the paper non-passage area becomes poorer than for the paperpassage area, and magnetic flux from exciting coil 120 acting uponfixing belt 112 in the paper non-passage area decreases. As a result,heat production distribution of the narrow paper non-passage areadeclines, and an excessive rise in the temperature of the papernon-passage area can be prevented.

Then, when the temperature measured by temperature sensors 541 and 542becomes a second predetermined temperature (for example, 160° C.) thatis lower than the fixing temperature, part D, part E, and part F arepositioned to face exciting coil 120 as shown in FIG. 21A, and uniformheat production distribution is thereby restored.

When medium-width paper is passed through in the state shown in FIG.21A, heat of only that paper passage area is absorbed by the recordingpaper, and together with this, temperature control is performed based ona temperature signal from temperature sensor 118. Therefore, thetemperature of the paper non-passage area (the range comprising themaximum-width paper passage range minus the medium-width paper passagerange) rises. When the temperature measured by temperature sensor 542exceeds 180° C., opposed core 116 c is rotated through 180 degrees andpart D, part E, and part f are positioned to face exciting coil 120 asshown in FIG. 21C. As the permeability of part f is lower than that ofpart D and part E, the magnetic coupling between fixing belt 112 andexciting coil 120 in the paper non-passage area becomes poorer than forthe paper passage area, and magnetic flux from exciting coil 120 actingupon fixing belt 112 in the paper non-passage area decreases. As aresult, heat production distribution of the medium-width papernon-passage area declines, and an excessive rise in the temperature ofthe paper non-passage area can be prevented.

Then, when the temperature measured by temperature sensor 542 becomes asecond predetermined temperature (for example, 160° C.) that is lowerthan the fixing temperature, part D, part E, and part F are positionedto face exciting coil 120 as shown in FIG. 21A, and uniform heatproduction distribution is thereby restored.

If a printing operation is performed with narrow paper when fixing unit19 is cold (for example, at room temperature), heating is started in thestate shown in FIG. 21C in order to heat only the paper passage area(medium-width paper passage range). In this case, since only the paperpassage area (medium-width paper passage range) is heated, the thermalcapacity for heating decreases. Therefore, the temperature can be raisedto the predetermined temperature (170° C.) with a small amount ofenergy, and if heating is performed with the same power, the temperaturecan be raised in a short time.

Also, since the temperature of fixing belt 112 in the paper non-passagearea does not rise to the fixing temperature, it is possible to preventthe temperature of pressure roller 115 in the paper non-passage areafrom becoming excessively higher than in the paper passage area.

On the other hand, a state is established in which the temperature oftemperature sensor 118 is higher than the temperature of temperaturesensor 542. If wide paper is next to be fed through in this state, it isnecessary to heat only the left-hand end part (the range comprising themaximum-width paper passage range minus the medium-width paper passagerange). In this case, part d, part e, and part F are positioned to faceexciting coil 120. In the heat production distribution in this state,the calorific value of the right-hand side (the medium-width paperpassage range) is small, and the calorific value of the left-hand side(the range comprising the maximum-width paper passage range minus themedium-width paper passage range) is large. By this means it is possibleto change from a state in which the temperature of the left-hand side islow to a state of uniform temperature distribution. At this time, thetemperature of the paper non-passage area of pressure roller 115 has notrisen excessively, and therefore, even when wide paper is fed through,it is possible to prevent irregularities such as uneven glossiness of afixed image caused by nonuniformity of the temperature of pressureroller 115, enabling high-quality images to be obtained.

As described above, according to this embodiment, the temperaturedistribution of fixing belt 112 can be kept constantly substantiallyuniform even when narrow paper or medium-width paper is continuously fedthrough. Therefore, fixing defects such as cold offset or hot offset dueto nonuniformity of fixing temperature distribution can be preventedwhen wide paper is fed through immediately after narrow paper is fedthrough, or when narrow paper, medium-width paper, and wide paper arefed through alternately. That is to say, if the entire width of fixingbelt 112 is heated during warm-up, it is possible to feed through bothnarrow paper and wide paper immediately. On the other hand, if only thenarrow paper passage range, for example, is heated during warm-up, inthe event of an abnormality such as non-rotation of fixing belt 112, thesurface temperature of fixing belt 112 will rise steeply and a safetymechanism (such as a thermostat, for example) may not be able to follow.If full-width heating is performed during warm-up, the temperature risetime of fixing belt 112 can be extended, and the follow-up operation ofthe safety mechanism can be assured.

Also, when starting up for narrow paper or medium-width paper printing,it is possible to heat only the paper passage area, enabling thetemperature to be raised with a small amount of energy, and alsoenabling the temperature to be raised in a short time if heating isperformed with the same power.

Furthermore, since opposed core 116 is divided in the axial directioninto plural parts each of which is rotatably configured, heating can beperformed using any combination of the right-hand, center, and left-handparts. Therefore, even if the temperature of an end part has become toolow relative to that of the center part due to heat dissipation to theend part or the like, uniform temperature distribution can be restoredby heating only that end part.

Moreover, as opposed core 116 has a uniform cross-sectional shape in theaxial direction, thermal capacity distribution of the heat-producingsection is uniform in the axial direction. Therefore, uniformtemperature distribution can easily be achieved by performing heatinguniformly by means of exciting coil 120.

A paramagnetic substance with a relative permeability of 1 or aconductor such as aluminum may also be used for low-permeability part d,part e, and part f.

Embodiment 6

FIG. 23 is a cross-sectional drawing of the principal parts of a fixingunit according to Embodiment 6 of the present invention, and FIG. 24 isa principal-part configuration drawing of opposed core 116 constitutinga magnetic flux adjustment section viewed from the direction indicatedby arrow H in FIG. 23.

This embodiment differs from Embodiment 3 in the configuration of themagnetic flux adjustment section. That is to say, in this embodiment,cylindrical opposed core 116 is regarded as a combination of twosemicylinders, one of which is called part a, and the other one part b.Opposed core 116 has a suppression member 650 that covers the part ofthe outer peripheral surface of part b that corresponds to the narrowpaper non-passage area. Suppression member 650 has an arc-shaped outerperipheral surface. Suppression member 650 is made of a nonmagneticconductive material such as aluminum. The distance between opposed core116 and the inner peripheral surface of retaining roller 113 is 0.6 mm,and the thickness of suppression member 650 is 0.3 mm.

Other details are similar to Embodiment 2, and configuration elements inthis embodiment that have the same action as in Embodiment 2 areassigned the same reference codes as in Embodiment 2, and detaileddescriptions thereof are omitted.

The operation and action of opposed core 116 as a magnetic fluxadjustment section in this embodiment will now be described.

If the temperature measured by end temperature sensor 232 is lower thana first predetermined temperature (for example, 180° C.) that is higherthan the fixing temperature (for example, 170° C.), part a of opposedcore 116 is positioned to face exciting coil 120. When current is passedthrough exciting coil 120 in this state, magnetic flux acts uniformlyupon the entire axial-direction width of fixing belt 112, and inductionheating is performed uniformly. When recording paper 16 fed through iswide, heat is absorbed over substantially the entire width, andtherefore the temperature of fixing belt 112 is maintained uniformlyover its entire width.

When narrow recording paper 16 is passed through, heat of only thecenter part is absorbed by opposed core 116, and together with this,temperature control is performed based on a temperature signal fromtemperature sensor 118. Therefore, the temperature of both end parts,which are paper non-passage areas, rises. When the temperature measuredby temperature sensor 232 exceeds 180° C., opposed core 116 is rotatedand part b is positioned to face exciting coil 120. That is to say,suppression member 650 is positioned between parts of fixing belt 112corresponding to paper non-passage areas and opposed core 116. In thisstate, an eddy current is induced in suppression member 650, andprevents fluctuation of the magnetic flux that permeates suppressionmember 650. Through this action, magnetic flux from exciting coil 120acting upon fixing belt 112 in the paper non-passage areas decreases.Consequently, the magnetic coupling between fixing belt 112 and excitingcoil 120 in the paper non-passage areas becomes poorer than for thepaper passage area. As a result, heat production distribution of thenarrow paper non-passage areas declines, and an excessive rise in thetemperature of the paper non-passage areas can be prevented.

Then, when the temperature measured by temperature sensor 232 becomes asecond predetermined temperature (for example, 160° C.) that is lowerthan the fixing temperature, part a of opposed core 116 is positioned toface exciting coil 120 and uniform heat production distribution isrestored.

As described above, according to this embodiment, the temperaturedistribution of fixing belt 112 can be kept constantly substantiallyuniform even when narrow paper is continuously fed through. Therefore,fixing defects such as cold offset or hot offset due to nonuniformity offixing temperature distribution can be prevented when wide paper is fedthrough immediately after narrow paper is fed through, or when narrowpaper and wide paper are fed through alternately. That is to say, if theentire width of fixing belt 112 is heated during warm-up, it is possibleto feed through both narrow paper and wide paper immediately. On theother hand, if only the narrow paper passage range, for example, isheated during warm-up, in the event of an abnormality such asnon-rotation of fixing belt 112, the surface temperature of fixing belt112 will rise steeply and a safety mechanism (such as a thermostat, forexample) may not be able to follow. If full-width heating is performedduring warm-up, the temperature rise time of fixing belt 112 can beextended, and the follow-up operation of the safety mechanism can beassured.

With a conventional fixing unit, if the temperature of both ends becomesexcessively high when narrow paper is fed through continuously, it isnecessary to stop the printing operation and wait until the temperatureof both ends falls, or to increase the recording paper feeding interval.With this embodiment, on the other hand, a rise in the temperature ofboth ends when narrow paper is fed through can be suppressed, making itunnecessary to wait or increase the paper feeding interval in the eventof an excessive rise in temperature. Therefore, throughput (the numberof sheets output per unit time) can be set high when outputting narrowpaper continuously.

As opposed core 116 is rotated as an integral unit, the mechanism forrotational drive is simple. In the case of a configuration in which thecenter part of the opposed core is fixed, a complex mechanism isnecessary in order to rotate only the end parts.

It is desirable for the conductive material of suppression member 650 tohave a volume resistivity of not more than 10×10⁻⁸ Ω·m to prevent heatgeneration due to induction heating. It is also desirable for itsthickness to be not less than 0.2 mm in order to prevent inductionheating. As the distance between opposed core 116 and fixing belt 112 atthe center is increased by the thickness of suppression member 650, thethinner suppression member 650 is the better. In order to securesufficient magnetic coupling between exciting coil 120, fixing belt 112,and opposed core 116, it is desirable for the thickness of suppressionmember 650 to be not more than 2 mm.

In this embodiment, the cross-sectional shape of opposed core 116 isuniformly cylindrical in the axial direction. However, the shape ofopposed core 116 is not limited to this. For example, a recess may beprovided in the part of the outer peripheral surface of part b ofopposed core 116 corresponding to a paper non-passage area, andsuppression member 650 may be fitted in this recess, as shown in FIG.25. Providing a recess in opposed core 116 in this way simplifiespositioning when fitting suppression member 650, and so simplifiesassembly. Suppression member 650 is fitted so that its outer peripheralsurface is located on the same circumferential surface as the outerperipheral surface of opposed core 116. By locating the outer peripheralsurface of opposed core 116 and the outer peripheral surface ofsuppression member 650 on the same circumferential surface in this way,the distance from retaining roller 113 to opposed core 116 and thedistance from retaining roller 113 to suppression member 650 are madeequal—that is, heat conduction from retaining roller 113 to opposed core116 and heat conduction from retaining roller 113 to suppression member650 are made equal—making it possible to prevent temperaturenonuniformity of fixing belt 112. Moreover, in this case, the distancebetween opposed core 116 and fixing belt 112 is reduced by the thicknessof suppression member 650, enabling magnetic coupling between excitingcoil 120, fixing belt 112, and opposed core 116 to be increased.

An opposed core 116 of the shape described in Embodiment 3 may be used,and suppression member 650 may be provided at both ends of this opposedcore 116 (parts having a semicylindrical shape), as shown in FIG. 26. Inthis case, the same kind of effect as described above can be obtained byhaving the outer peripheral surface of suppression member 650 located onthe same circumferential surface as the outer peripheral surface ofopposed core 116. The same kind of effect can also be obtained ifsuppression member 650 is a hollow semicylinder as shown in FIG. 27.

Suppression member 650 shown in FIG. 28 has a configuration in which aprojection 650 a is provided at either circumferential end ofsuppression member 650 shown in FIG. 25. This enables wraparound ofmagnetic flux reaching front cores 121 c from center core 121 a viafixing belt 112 to be suppressed, enabling heat production to be reducedeffectively.

It is also possible to apply a combination of opposed core 116 andsuppression member 650 described in this embodiment to the configurationdescribed using FIG. 12 in Embodiment 1, as shown in FIG. 29. The samekind of effect can be achieved in this case as with the configurationshown in FIG. 12.

In the prior art, suppression member 650 has been located betweenexciting coil 120 and fixing belt 112. In this embodiment, on the otherhand, suppression member 650 is located on the opposite side of fixingbelt 112 from exciting coil 120. By this means, the current and voltageinduced in the suppression coil are made small, and a rise in thetemperature of suppression member 650 is suppressed. Also, the thicknessof suppression member 650 does not affect the distance between fixingbelt 112 and exciting coil 120, enabling suppression member 650 to bemade fully as thick as necessary. Furthermore, as suppression member 650is attached to opposed core 116 made of ferrite that has thermalconductivity of a predetermined level or above, heat dissipation fromsuppression member 650 can be performed efficiently. That is to say,from these points of view, it can be said to be possible to suppress arise in the temperature of suppression member 650. As a result,induction heating energy consumed by suppression member 650 can besuppressed, enabling thermal efficiency for heating fixing belt 112 tobe improved and temperature rises of suppression member 650 to besuppressed, thereby making it possible to perform continuous feeding ofnarrow paper.

In this embodiment, opposed core 116 is assumed to be an integral unit,but opposed core 116 may also be divided in the axial direction in thesame way as in Embodiment 5.

In the above embodiment, the rotational phase of opposed core 116 isswitched based on a temperature signal from temperature sensor 232.However, the basis for phase switching is not limited to this, and therotational phase may also be switched according to the width ofrecording paper 16, for example.

Embodiment 7

FIG. 30 is a cross-sectional drawing of the principal parts of a fixingunit according to Embodiment 7 of the present invention, and FIG. 31 isa principal-part configuration drawing along line J-J of an opposed coreconstituting a magnetic flux adjustment section in the fixing in FIG.30.

This embodiment differs from Embodiment 6 in the configuration of fixingunit 19. That is to say, as shown in the drawings, exciting coil 120 isinstalled inside retaining roller 113, retaining roller 113 is pressedagainst pressure roller 115 via fixing belt 112, and an approximatelyarc-shaped suppression member 750 is positioned to closely face theouter peripheral surface of fixing belt 112.

Suppression member 750 is divided into three in the axial direction,being composed of a suppression member 750 a and two suppression members750 b. Suppression member 750 a is located in the center in the axialdirection, and suppression members 750 b are located at either side inthe axial direction. The division locations correspond to the two edgesof a predetermined narrow paper passage range. Suppression member 750 ismade of 1.5 mm thick aluminum sheet. Suppression members 750 a and 750 bare supported movably in the radial direction of fixing belt 112.Suppression members 750 a and 750 b are displaced between a nearposition at a distance of 0.5 mm from fixing belt 112 and a far positionat a distance of 4 mm from fixing belt 112.

Other details are similar to Embodiment 6, and configuration elements inthis embodiment that have the same action as in Embodiment 6 areassigned the same reference codes as in Embodiment 6, and detaileddescriptions thereof are omitted.

The operation and action of suppression member 750 as a magnetic fluxadjustment section in this embodiment will now be described.

If the temperature difference between center temperature sensor 118 andend temperature sensor 232 is less than a predetermined temperaturedifference (for example, 15° C.), and the temperature measured bytemperature sensor 232 is lower than a first predetermined temperature(for example, 180° C.) that is higher than the fixing temperature (forexample, 170° C.), both suppression member 750 a and suppression members750 b are displaced to the far positions shown by the dotted lines inFIG. 31. When current is passed through exciting coil 120 in this state,magnetic flux acts uniformly upon the entire axial-direction width offixing belt 112, and induction heating is performed uniformly. Whenrecording paper 16 fed through is wide, heat is absorbed oversubstantially the entire width, and therefore the temperature of fixingbelt 112 is maintained uniformly over its entire width.

When narrow recording paper 16 is passed through in this state, heat ofonly the center part is absorbed by the recording paper, and togetherwith this, temperature control is performed based on a temperaturesignal from center temperature sensor 118. Therefore, the temperature ofboth end parts, which are paper non-passage areas, rises. When thetemperature measured by temperature sensor 232 exceeds 180° C.,suppression members 750 b at both ends are displaced to the nearpositions shown by the solid lines in FIG. 31. With these suppressionmembers 750 b at both ends brought near to fixing belt 112, the magneticcoupling between fixing belt 112 and exciting coil 120 in the papernon-passage areas becomes poorer than for the paper passage area, andmagnetic flux from exciting coil 120 acting upon fixing belt 112 in thepaper non-passage areas decreases. As a result, heat productiondistribution of the narrow paper non-passage areas declines, and anexcessive rise in the temperature of the paper non-passage areas can beprevented.

Then, when the temperature measured by temperature sensor 232 becomes asecond predetermined temperature (for example, 160° C.) that is lowerthan the fixing temperature, suppression members 750 b at both ends aremoved to their far positions and uniform heat production distribution isrestored.

If a printing operation is performed with narrow paper when fixing unit19 is cold (for example, at room temperature), heating is started withsuppression members 750 b at both ends in their near positions. At thistime, since only the center part is heated with high intensity heatproduction distribution, the thermal capacity for heating decreases.Therefore, the temperature can be raised to the predeterminedtemperature (170° C.) with a small amount of energy, and if heating isperformed with the same power, the temperature can be raised in a shorttime.

At this time, also, a state is established in which the temperature ofcenter temperature sensor 118 is higher than that of end temperaturesensor 232. If wide paper is next to be fed through in this state, it isnecessary to heat only both end parts. For example, when the temperaturedifference between temperature sensor 118 and temperature sensor 232reaches a predetermined value (for example, 15° C.) or more, center-partsuppression member 750 a is displaced to the near position and end-partsuppression members 750 b are displaced to the far position. In the heatproduction distribution in this state, the calorific value of the centerpart is small, and the calorific value of the end parts is large. Bythis means it is possible to change from a state in which thetemperature of the end parts is low to a state of uniform temperaturedistribution.

Also, by locating electrically conductive suppression member 750 on theoutside of fixing belt 112, leakage of magnetic flux outside fixing unit19 can be prevented.

As described above, according to this embodiment, the temperaturedistribution of fixing belt 112 can be kept constantly substantiallyuniform even when narrow paper is continuously fed through. Therefore,fixing defects such as cold offset or hot offset due to nonuniformity offixing temperature distribution can be prevented when wide paper is fedthrough immediately after narrow paper is fed through, or when narrowpaper and wide paper are fed through alternately. That is to say, if theentire width of fixing belt 112 is heated during warm-up, it is possibleto feed through both narrow paper and wide paper immediately. On theother hand, if only the narrow paper passage range, for example, isheated during warm-up, in the event of an abnormality such asnon-rotation of fixing belt 112, the surface temperature of fixing belt112 will rise steeply and a safety mechanism (such as a thermostat, forexample) may not be able to follow. If full-width heating is performedduring warm-up, the temperature rise time of fixing belt 112 can beextended, and the follow-up operation of the safety mechanism can beassured.

Also, when starting up for narrow paper printing, it is possible to heatonly the center part, enabling the temperature to be raised with a smallamount of energy, and also enabling the temperature to be raised in ashort time if heating is performed with the same power. Furthermore,uniform temperature distribution can be restored even when thetemperature of the end parts has become too low relative to that of thecenter part due to heat dissipation to the end parts or the like.

In the prior art, suppression member 750 has been located betweenexciting coil 120 and fixing belt 112. In this embodiment, on the otherhand, suppression member 750 is located on the opposite side of fixingbelt 112 from exciting coil 120. By this means, the current and voltageinduced in the suppression coil are made small, and a rise in thetemperature of suppression member 750 is suppressed. Also, the thicknessof suppression member 750 does not affect the distance between fixingbelt 112 and exciting coil 120, enabling suppression member 750 to bemade fully as thick as necessary. That is to say, from this point ofview, it can be said to be possible to suppress arise in the temperatureof suppression member 750. As a result, induction heating energyconsumed by suppression member 750 can be suppressed, enabling thermalefficiency for heating fixing belt 112 to be improved and temperaturerises of suppression member 750 to be suppressed, thereby making itpossible to perform continuous feeding of narrow paper.

In this embodiment, suppression member 750 is configured movably in theradial direction of fixing belt 112, but suppression member 750 is notlimited to this configuration. For example, two suppression members 750b that are movable in the axial direction may be provided at both endparts comprising paper non-passage areas—that is, the ranges resultingfrom excluding the narrow paper passage range from the maximum-widthpaper passage range—as shown in FIG. 32 and FIG. 33. In FIG. 33, opposedcore 116 is located on the opposite side of suppression members 750 bfrom fixing belt 112.

In the case of this configuration, suppression members 750 b are movedoutside the maximum-width paper passage range to provide uniform heatproduction distribution, and are moved to positions corresponding to thewidth of recording paper 16 fed through to lower the heat productiondistribution of the paper non-passage areas at both ends. By this means,the width of low heat production distribution areas can be setcontinuously and arbitrarily. As a result, an excessive rise in thetemperature of paper non-passage areas can be prevented for any width ofrecording paper 16 fed through.

The configuration of fixing unit 19 of the present invention is notlimited to the above-described configurations, and application is alsopossible to cases where exciting coil 120 is provided on both the outerperipheral surface side and inner peripheral surface side of fixing belt112.

As described above, according to the present invention the entire widthof a heat-producing member can be heated uniformly and an excessive risein the temperature of the heat-producing member can be prevented,without making the configuration complex.

This application is based on Japanese Patent Application No. 2003-002058filed on Jan. 18, 2003, the entire content of which is expresslyincorporated by reference herein.

INDUSTRIAL APPLICABILITY

An image heating apparatus and image forming apparatus of the presentinvention have an effect of heating the entire width of a heat-producingmember uniformly and preventing an excessive rise in the temperature ofthe heat-producing member, without making the configuration complex, andare useful as an image heating apparatus using an electromagneticinduction heating scheme for fixing an unfixed image, and an imageforming apparatus such as an electrophotographic apparatus orelectrostatographic apparatus.

1. An image heating apparatus comprising: an annular heat-producing member that has a pair of principal surfaces and produces heat through the action of magnetic flux; a magnetic flux generation section that is located in proximity to a first principal surface of said pair of principal surfaces and generates magnetic flux that acts upon said heat-producing member; and a magnetic flux reduction section that is located in proximity to a second principal surface of said pair of principal surfaces and reduces, of magnetic flux generated by said magnetic flux generation section, magnetic flux that acts upon a paper non-passage area of said heat-producing member.
 2. The image heating apparatus according to claim 1, wherein said magnetic flux reduction section is attached to a rotatable core and is displaced between a facing position with respect to said second principal surface and a non-facing position with respect to said second principal surface by rotating said core.
 3. The image heating apparatus according to claim 2, wherein: said core has a recess formed in an outer peripheral surface thereof; and said magnetic flux reduction section is attached by being inserted in said recess.
 4. The image heating apparatus according to claim 2, wherein said magnetic flux reduction section is attached to said core so that an outer peripheral surface thereof is located on a circumferential surface the same as an outer peripheral surface of said core.
 5. The image heating apparatus according to claim 2, wherein said magnetic flux reduction section has a projection formed on a peripheral-direction end thereof.
 6. The image heating apparatus according to claim 2, wherein said magnetic flux reduction section is displaced to said non-facing position at warm-up time.
 7. The image heating apparatus according to claim 2, wherein said magnetic flux reduction section is first displaced to said non-facing position and thereafter displaced to said facing position when sheets narrower than a maximum-width paper passage range are passed through continuously.
 8. The image heating apparatus according to claim 2, wherein said core has thermal conductivity of a predetermined level or above.
 9. An image forming apparatus that has the image heating apparatus according to claim
 1. 10. An image heating apparatus comprising: an induction-heated thin heat-producing member that transfers heat directly or indirectly to a heated medium that moves with an image; an excitation section that generates magnetic flux and induction-heats said heat-producing member; a temperature control section that controls said excitation section and makes a temperature of a contact surface that comes into contact with said heated medium a predetermined temperature; and a heat production adjustment section that is located on an opposite side of said heat-producing member relative to said excitation section, and adjusts heat production distribution of said heat-producing member by adjusting magnetic flux acting upon said heat-producing member; wherein said heat production adjustment section has an opposed core of ferromagnetic material whose temperature varies according to a temperature of said heat-producing member and whose Curie point is in a range of −10° C. to +100° C. relative to a maximum value of said predetermined temperature.
 11. An image heating apparatus comprising: an induction-heated thin heat-producing member that transfers heat directly or indirectly to a heated medium that moves with an image; an excitation section that generates magnetic flux and induction-heats said heat-producing member; a temperature control section that controls said excitation section and makes a temperature of a fixing surface that comes into contact with said heated medium a predetermined temperature; and a heat production adjustment section that is located on an opposite side of said heat-producing member relative to said excitation section, and adjusts heat production distribution of said heat-producing member by adjusting magnetic flux acting upon said heat-producing member; wherein said heat production adjustment section has an opposed core of ferromagnetic material whose temperature varies according to a temperature of said heat-producing member and whose Curie point is in a range of 140° C. to 250° C.
 12. The image heating apparatus according to claim 10, wherein said heat production adjustment section comes into contact with said heat-producing member or a member heated by said heat-producing member.
 13. An image heating apparatus comprising: an induction-heated thin heat-producing member that transfers heat directly or indirectly to a heated medium that moves with an image; an excitation section that generates magnetic flux and induction-heats said heat-producing member; a temperature control section that controls said excitation section and makes a temperature of a fixing surface that comes into contact with said heated medium a predetermined temperature; and a heat production adjustment section that is located on an opposite side of said heat-producing member relative to said excitation section, and adjusts heat production distribution of said heat-producing member by adjusting magnetic flux acting upon said heat-producing member; wherein said heat production adjustment section has a switching section that performs on/off switching of a suppression coil composed of an electrical conductor that is linked to said magnetic flux.
 14. The image heating apparatus according to claim 13, wherein an opposed core of high-permeability material that is permeated by magnetic flux linked to said suppression coil is located inside said suppression coil or on an opposite side of said heat-producing member relative to said suppression coil.
 15. An image heating apparatus comprising: an induction-heated thin, cylindrical heat-producing member that transfers heat directly or indirectly to a heated medium that moves with an image; an excitation section that faces an outer peripheral surface of said heat-producing member, generates magnetic flux, and induction-heats said heat-producing member; a temperature control section that controls said excitation section and makes a temperature of a fixing surface that comes into contact with said heated medium a predetermined temperature; and a heat production adjustment section that is located on an opposite side of said heat-producing member relative to said excitation section, and adjusts heat production distribution of said heat-producing member by adjusting magnetic flux acting upon said heat-producing member; wherein said heat production adjustment section has a rotatable opposed core of ferromagnetic material whose cross-sectional shape varies in an axial direction of said heat-producing member.
 16. The image heating apparatus according to claim 15, wherein a distance between said heat-producing member and said opposed core is fixed in an axial direction in at least one part of a circumferential direction of said opposed core.
 17. The image heating apparatus according to claim 15, wherein heat production distribution whereby intensity of heat production distribution adjusted by said opposed core is reversed by rotation of said opposed core is possible.
 18. The image heating apparatus according to claim 15, wherein said opposed core is formed by combining a plurality of materials of different permeability in at least one part of an axial direction of said heat-producing member.
 19. The image heating apparatus according to claim 18, wherein said opposed core is formed by combining at least a ferromagnetic material and a low-permeability electrical conductor.
 20. The image heating apparatus according to claim 15, wherein a cross-sectional shape of said opposed core varies continuously in an axial direction in at least one part of an axial direction of said heat-producing member.
 21. An image heating apparatus comprising: an induction-heated thin, cylindrical heat-producing member that transfers heat directly or indirectly to a heated medium that moves with an image; an excitation section that faces an outer peripheral surface of said heat-producing member, generates magnetic flux, and induction-heats said heat-producing member; a temperature control section that controls said excitation section and makes a temperature of a fixing surface that comes into contact with said heated medium a predetermined temperature; and a heat production adjustment section that is located on an opposite side of said heat-producing member relative to said excitation section, and adjusts heat production distribution of said heat-producing member by adjusting magnetic flux acting upon said heat-producing member; wherein said heat production adjustment section has a rotatable opposed core which consists of divided ferromagnetic materials and has a cross-sectional shape varying in an axial direction of said heat-producing member.
 22. The image heating apparatus according to claim 21, wherein said opposed core is formed by combining a plurality of materials of different permeability in at least one part of an axial direction of said heat-producing member.
 23. The image heating apparatus according to claim 21, wherein said opposed core is formed by combining at least a ferromagnetic material and a low-permeability electrical conductor.
 24. The image heating apparatus according to claim 21, wherein a cross-sectional shape of said opposed core varies continuously in an axial direction in at least one part of an axial direction of said heat-producing member.
 25. An image heating apparatus comprising: an induction-heated thin, cylindrical heat-producing member that transfers heat directly or indirectly to a heated medium that moves with an image; an excitation section that generates magnetic flux and induction-heats said heat-producing member; a temperature control section that controls said excitation section and makes a temperature of a fixing surface that comes into contact with said heated medium a predetermined temperature; and a heat production adjustment section that is located on an opposite side of said heat-producing member relative to said excitation section, and adjusts heat production distribution of said heat-producing member by adjusting magnetic flux acting upon said heat-producing member; wherein said heat production adjustment section has a movable magnetic flux suppression member of low-resistivity material.
 26. The image heating apparatus according to claim 25, wherein in said heat production adjustment section said magnetic flux suppression member is attached to an opposed core of ferromagnetic material.
 27. An image heating apparatus comprising: an induction-heated thin heat-producing member that transfers heat directly or indirectly to a heated medium that moves with an image; an excitation section that generates magnetic flux and induction-heats said heat-producing member; a temperature control section that controls said excitation section and makes a temperature of a contact surface that comes into contact with said heated medium a predetermined temperature; and a heat production adjustment section that is located on an opposite side of said heat-producing member relative to said excitation section, and adjusts heat production distribution of said heat-producing member by adjusting magnetic flux acting upon said heat-producing member; wherein said heat production adjustment section has an opposed core of ferromagnetic material, and a Curie point of said opposed core is set higher than a temperature of said opposed core in a paper passage area and lower than a temperature of said opposed core in a paper non-passage area.
 28. An image heating apparatus comprising: an induction-heated thin heat-producing member that transfers heat directly or indirectly to a heated medium that moves with an image; an excitation section that generates magnetic flux and induction-heats said heat-producing member; a temperature control section that controls said excitation section and makes a temperature of a contact surface that comes into contact with said heated medium a predetermined temperature; and an opposed core of ferromagnetic material that is located on an opposite side of said heat-producing member relative to said excitation section, and adjusts heat production distribution of said heat-producing member by adjusting magnetic flux acting upon said heat-producing member; wherein a distance between said heat-producing member and said opposed core is set constant in an area facing said exciting section.
 29. An image heating apparatus comprising: an induction-heated thin heat-producing member that transfers heat directly or indirectly to a heated medium that moves with an image; an excitation section that generates magnetic flux and induction-heats said heat-producing member; a temperature control section that controls said excitation section and makes a temperature of a contact surface that comes into contact with said heated medium a predetermined temperature; and an opposed core of ferromagnetic material that is located on an opposite side of said heat-producing member relative to said excitation section, and adjusts heat production distribution of said heat-producing member by adjusting magnetic flux acting upon said heat-producing member; wherein a distance between said heat-producing member and said opposed core in a paper non-passage area is set greater than a distance between said heat-producing member and said opposed core in a paper passage area.
 30. An image forming apparatus comprising: the image heating apparatus of claim 10; and a temperature sensor that measures a temperature of a paper non-passage area of said heat-producing member; wherein a heat production adjustment section adjusts heat production distribution of said heat-producing member based on a signal from said temperature sensor. 